Sorry, I don't have a set of rollers nor do I have a range for wheel base on different bikes, but you could possibly glue on either a veneer onto the PVC if you went down that road. I personally love the idea of a solid block of maple or oak.

I have seen rollers advertised as having curved rollers to help center, but I have no idea how much they are curved.

Just curiously, have you done any calculations on roller diameter, speed and load on the bearings you are thinking of? I did a quick calculation and it would seem that with a 2" diameter roller going biking at 20 mph the roller (and thus bearing) would be spinning at 3300 rpm. You might want to make sure that the bearings you select and the diameter of the roller will work well with the expected speed that you will be riding.

Can anyone confirm my calculation? Also, it seems like a larger roller would be quieter as it would spin slower.