Do you need flat guides?
My first choice would be round guide rod and V-rollers, bit like this:
http://www.pbclinear.com/LPRR-Low-Profile-Redi-Rail-Linear-Guide?utm_source=RelatedItem&utm_medium=Internal&utm_campaign=Related%2BItemOne linear actuator maker has othervise similar structure, but large V-runners are staggered inside and guide rods are outside. Compact structure and relatively large contact area without many rollers.
Rollers must be V:
http://www.pbclinear.com/Images/Items/prod_crt_V-Guide_Wheels.jpgRound approximation is really bad idea, I should know.
Ok, you may need several runners because guide has a line contact and runner two point contacts, but there are benefits:
* Dirt will be squeezed out, instead of rolled onto guide.
* Easier to align than rectangular one and easier to get hardened and true
* Possible to dab a hard surface away with a grinding disc and then drill hole trough softer part and still preserve hard/true running surfaces.
You need hardened guides, soft ones will dent/scratch/wear way too fast. One machine had rolled square pipe that had corners machined and although it was stiff, lightweight and accurate while test run, it did not last in real production environment.
My 5 eurocents,
Pekka
