How about a Belt Sander Belt?
They are cheap, and lots of sizes to choose from. Has anyone considered this already? Pros? Cons? Was it dismissed for any reason?
Here’s an example of what I am thinking of. 16" width. 48" length, which is the length of it wrapped around your pulleys, so this would support a roughly 20" center to center distance of your printer pulleys. Seems like a nice size. ~$31/ea
and a 10" width but ~70" long. ~$20-30/ ea
Other size options exist on McMaster. May be better deals or options down at the local hardware store.
One benefit with using a sanding belt is that the abrasive material can be used to add friction between the belt and the pulley, requiring less tension on the belt. However, the abrasive side would then be scrubbing over your build plate and would eventually wear it down, not to mention contantly generating debris. One mitigation for this would be to use UHMC (used a a slippery wear surface in industrial material handling chutes), but you’d [likely] loose any option for a heated build plate. Maybe not a problem if you stick to PLA or PETG and have a built tack surface. But, perhaps a better option would be to put the abrasive side pointing up towards the extruder, and with a high grit (for a smoother surface), stick down some build tack (or other sticky surface)?
Let me know your thoughts!