Been scheming a way to build this for awhile, trying to decide how tall/wide/deep...etc I wanted to build it. Then, trying to decide how exactly I'd attach it to the truck, which rollers to use, materials, etc.
This is what I came up with The "roller-base" does not attach, per-se... the sides are beveled slightly so that it wedges in between the wheel-wells. You cannot slide it out, it must be lifted, then removed. It's made out of 5/8" plywood, then coated with bedliner. The rollers are "ball-bearing rollers"... I've been trying to find them for awhile, and finally found them at Harbor Freight (gotta love that store!) for around $1/ea.
The tailgate piece was made (with that lip/raised area) to fit securely against the tray, overlapping the edges, and forming a tight seal. It's attached to the tailgate with industrial strength velcro. It's covered in bedliner spray. It also acts to protect the tailgate/paint...
The tray is made out of 3/4" plywood... built using heavy-duty screws, and of course covered with bedliner. It's 48" wide X 65" long X 9" deep (interior). I was able to find a cheap piece of carpet to line the bottom. It has divider/inserts that can be used when desired, as well as a middle support divider in case I need to load anything heavy on the top cover pieces.
The cover is 3-pieces of 3/4" plywood (bedliner spray), and each piece is 60" wide X 21.5" deep. The pieces have "guides" on the bottom so they sit perfectly centered on the tray. The 1st piece has a metal-tab that secures it to the rear of the tray. Then, each piece has metal tabs that secure them to eachother, so that none can be lifted off without first rolling out the tray & removing the 3rd cover piece first, then the 2nd, then the last. Each has 2 cargo-hooks bolted in. When the tray is rolled in, the pieces cannot be opened or removed due to the guide-wheels on top (you'll see them on each side of the bed-opening). These have 2 functions: 1) to lock down the covers, 2) so that when you roll-out the tray, the wheels keep it from tipping down.
The unit is heavy... I'd say that the total added weight is around 200-250 lbs, maybe more. It's rugged, sturdy, and most of all, functional! What do ya think?