UPDATED!!!!! I covered it with truck carpet. See the end...
NO, it's not waterproof, and NO it's not as secure as a lock box, but it's sufficient to keep prying eyes out of my cargo area, carry my golf clubs or packages from the mall. I used existing holes & only spent $48.
$ 9 one 8' sheet of 5/8" plywood
$26 four 24" angle iron w/ holes
$ 8 one quart of flat black exterior paint from Walmart
$ 5 various nuts, bolts, washers & a can of Rustolem High Heat Enamel
$48 total cost to build this trunk!!
I had the guy at Lowes cut the 8' sheet of plywood down to 5' x 4' and 3' x 4' sections. You'll only need the 5' section to make all 3 pieces.
Step 1 - paint the parts (those metal brackets get HOT in the sun!!)
Step 2 - pull back the carpet & put one side under the side rails
Step 3 - use the 2 rear most holes that your hard top bolts into & fasten the angle iron to the vehicle. I used 1.5" 1/4" bolts, washers and a rubber washer that sits on the paint to prevent scratches
Step 4 - get another piece of angle iron & face it the opposite way, use 2 1/4" bolts to fasten it together. This will be the support for the cargo cover.
Step 5 - I cut 2 pieces for the top, just for ease of lifting it in & out. Step 5 is cut the piece that backs up to the rear seat.
View from underneath/inside the cargo area (yes I have a subwoofer):
Step 6 - cut the 2nd piece to fit the top.
I cut mine to fit WITH the bar that attaches to the rear window.
Step 7 - With both top pieces in place, go to the underside of the planks & use a marker to color in the holes thru the metal brackets. This will show you where to drill the holes.
Step 8 - Remove the planks, drill holes from the bottom side. Place them back onto the brackets & hopefully your bolts will slide into place from the top, thru the wood & thru the bracket. Secure w/ nuts.
Step 9 - cut out the back piece that will go behind the rear seats.
Step 10 - Secure with brackets from inside the cargo area
Step 11 - Once everything fits perfectly with bolts, remove & paint. I applied 2 coats.
Step 12 - Reassemble & all a cargo net if you like! It's pretty tough, but I wouldn't stand on it. I didn't want to have to cut metal to make a brace across the middle, but you could easily add a 5' length of bar across the middle, or whatever. From this, I'm sure you guys can modify & improve it.
UPDATED - STEP 13!!!
Step 13 - One $8 roll of truck carpet from Walmart (found in the floor mat section) & a can of spray adhesive. I covered it with the carpet & left the brackets exposed so that I can attach a cargo net or bungee cords on top if needed. You could cover it if you like...whatever suits your needs. I did, however, cover the rear edge so that it won't cut the soft top when it's folded down.