I made this a couple of years ago out of materials on hand. I had a look at a ready- made unit, but the cost was $800 and it was very flimsy. The salesman demonstrated how it could handle sliding in and out with 8 concrete construction blocks on it. I think if he had added one more it would have collapsed!

The ready-made unit had little 2" rails and bearings with an od. of about 1.5"

I used some thin wall 4" rails and 3.5" bearings. The maximum load I have placed on it fully extended was about 1/2 ton of cement gravel. It also got loaded with a similar amount of pasture fertilzer last month. Even with a half ton on there it slides in or out easily.
The only item purchased was one sheet of 3/4" fir plywood for the deck.

The extender and rails are easily removed from the truck (no bolts) but I find it so handy to have it in there that I rarely remove it.

Glen