Hi Trevor

The wheels Ive got to build rollers are about (guessing) 250mm in diameter and about 150mm wide, you can get them from disposal stores and engineering supply stores, they have a hard plastic rim, and metal bearing arrangement, if you knock out/punch out the bearing, this leaves about a 50mm hole with the slight taper in the middle, file the taper down flat, so a piece of 50mm aluminium tube can be pushed through, a couple of sleeves each side to hold the wheel in the one place and attach the cradles, most of these type (that I have seen) have wheels on the inside, but I've seen one with them on the outside.

PS the cheapest Ive seen the same wheels as I have is $40 at a market.

Regards