For what it is worth, the Woodmaster brand sanders use a 6 inch steel drum, they use the velcro sandpaper and the drums turn at 750 rpm, which they say is what 3M recommends for the sandpaper.
Wish I could afford one of theirs, they look quite solidly built.
I want to make one about 18 inches wide. My concern here is the power. I hope to use a 1.5 hp 1725 rpm motor, on 220v.
I also find these to be fairly expensive new, like $185 with shipping, I think eBay may have to be the answer. I do have some motors that I left at a friends shop about 17 years ago, he still has them and more, told me to come on over, so I hope there is something there I can use.
If you have to buy a motor for almost $200, and add all the other stuff, it makes the Jet/Performax 10-20inch model look pretty affordable.
I downloaded the plan from Shopnotes and it is pretty nice. I do agree the conveyor looks like it could wind up taking longer to build and adjust than the rest of the sander. I am thinking of a conveyance board, like a board that is flat and has a wide sanding belt of about 80 grit fitted around it, that rides on rails on the side of the table for instrument tops, you could even have a long handle on the back side and set it in place walk around and just pull it through. The table could be used without it for longer pieces.
I am not planning to build this for a while, just looking at the feasibility. Maybe someone will have the jet sander on sale.

I did buy an aluminum tube, 4" ID, 4.5" OD from someone on eBay. I have a lathe so I can make some trued discs to go into the tube. I also like the idea of the velcro paper, makes it easier to change the grit quickly and it keeps the paper cooler, which will make it last longer.