Ok, so a request has been made that I publish a plan, BOM, and instructions on building the cargo box; well here it is in Google Sketchup format, along with views to illustrate construction detail:
This basic box design is shrinkable down to about a 24" long box (any smaller, and you'll want to go to thinner wood), and extensible to about a 4'x8' box; but anything larger than 2'x6' is going to need extra bracing. Instead of just face framing the rim of the box, at six feet, you would add a center strip; and at 8 feet you would add two framing strips, one each 2 feet in from the edges.
BOM for a 60"x20"x24" box:
2x sheets, sanded A-A hard wood plywood (plenty of leftover) - Oak ply appx $40 ea x2 = $80
8x 1"x2"x10' hard wood strips - Red oak appx $8 each x6 = $64 (should be leftover, 8 is just in case)
1x 48" Piano hinge - appx $8 (alternately 4 door or cabinet hinges could be used)
1x padlock hasp - appx $4
2x sash lock - appx $4 ea x2 = $8
2x 28" lengths of a fine chain (used to limit the travel of the top of the box on its hinge)
Fasteners, finish etc...
50x 18ga 3/4" brads
100x 18ga 1" brads
100x 18ga 1-3/8" brads
1x bottle gorilla glue (appx 2oz used)
1x bottle (8oz) resin wood glue
1x can (4oz) Minwax Colonial Walnut (23) "wood finish"
1x can (8oz) marine spar varnish
The total materials costs were around $175 plus or minus a bit including fasteners and finish; but you could substitute cheaper A-C modified plywood and strip poplar and end up at under $120. Also, unless you screw up a lot of cuts, you're going to have plenty of left overs; but they'll be of odd lengths and sizes.
Total construction time including finishing was about 12 hours; but should have been about 8 (I screwed up some of my setups and had to reset them several times).
1. Plane the strips down to 1-1/2" x 1/2" exactly
2. Crosscut and rip the ply panels to length using a fine finish plywood blade, and a zero kerf insert, to avoid tearout
3. Run a 3/8" wide rabbett 3/8" deep around all the panels.
4. Using a corner clamp, glue and inset lap the two ends to the front face panel using Gorilla glue or a similar waterproof expanding glue; then cross nail with brads. Repeat for the second side, and then the back. In an inset lap, the front face should completely cover the end grain of the side face when place in position.
5. Flip the box over, careful not to chip the top. Run a gorilla glue bead around the bottom rabbet, and drop the bottom into place, cross nailing once it's in position.
6. Frame miter the oak strips to exactly match the outside dimensions of the box (top front, top sides, and top back face;bottom front, sides, and back face).
You are essentially building two 2" high 1/2" thick picture frames, exactly fitted to the top and bottom rims of the box, and flush and level (if you cant do both, make it level, just below the high spots in the plywood; and we''ll sand flush later).
Glue and brad the face frames in place, including cross nailing into the end grains of the miters. You may want to biscuit, dowel, or joint lock (a specially shaped piece of wood or fastener) the miters instead.
7. Cut the two front, two left side, two right side, and two rear vertical risers; trimmed to fit exactly into the space between the face frames, with a gentle tap with a mallet.
Fit the front and rear face frame risers flush with the outside edge of the top and bottom miter frames, and glue and brad in place. Butt the side face frame risers into the front and rear risers. Glue and brad in place, including cross nailing through the front face, and top rim.
8. Sand the top rim of the box flush and level.
9. place the top on the box, and measure the width from the edge of the top to the outside face of the oak strip frame. It should be 1/2" all the way around. Plane 14 feet of oak strip down to 3/8" thick, and rip in half. then plane the strips to 1/2" x 3/8".
The object here is to end up with a filler strip exactly flush to the top and face frame of the box, and long enough to completely surround it, with miters.
10. glue and brad (use the short brads) the filler strip down to the remaining oak top face frame strips, building an L shaped frame strip.
Alternatively you COULD start with 1.5" or 2" thick stock, then plane and rabbet it down. This would be stronger, and more attractive, but expensive and a lot of work.
11. Using this l shaped frame strip, build a flat mitered frame, butting the filler strip carefully flush to the top rim of the box face frame all the way around; then glue and cross nail it into place.
It is critical that the front and rear frame of the top, be absolutely flush with the face frame of the box; to mount hinges and closures.
Optionally, at this point you may wish to pre-drill and drive a 1" screw into all of the face frame joints, plus one at each end and the middle of the face frame joins to the plywood. (meaning each of the rim strips would have 5 screws - 1 in each miter, 1 at each end of the plywood, and one in the middle of the strip)
12. Allow the glue to dry overnight.
13. Rough and finish sand the entire box, inside and out. Scraping away any excess glue; and rounding all sharp corners.
14. Fill all gaps, cracks, and nail holes with putty filler, or oak flour (you should have enough of it by now) mixed into a filler with thick resin glue, and let dry.
15. Hand rub the entire box, evenly, inside and out with your stain or tinted finish sealer; then buff off after a few minutes. Let dry, and then lightly sand. and tack cloth away the dust. Add another coat if desired.
16. Apply at least three coats of marine spar varnish, or similar water resistant finish; lightly sanding between each coat.
17. Mount the hardware, and test function.