Hi Jak100
(hey, there's a pun there ;0 )
You'd want to be using fully hardened shaft in those linear ball bearings, if that is a consideration. All those tiny points of ball contact do not actually amount to a "good bearing" at all, even though the friction may be low, and providing you have such an exacting fit that they are practically preloaded.
Plastic is a bit draggy as you pointed out.
I'd use Oilite bushings, running on Chrome plated shafting, although it may be a trick to fasten the bushings into an opensided housing. However, working through the open side of the bearing housing, I think a guy could countersink a flathead screw into the bushing to retain it from the inside.
I would also maximize the length (through design) between bearings running on any one rail to minimize slew and jamming.
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