I covered many possibilities in my book “Setting-up a Small Observatory’, (Patrick Moore’s Practical Astronomy Library), still available from Springer, with descriptions and photos of methods of construction various people have used.
I used treated timber rails and guides and plastic rubber-tyred wheels, and, about 12 years on, this is the one part of my construction that has failed and needs replacing. The problem has been that wear by the wheels (even though there are lots of them, spreading the load widely) caused groove formation on the rails, which allowed water to stand, and the eventual decay of the rails. I build the shed in a modular fashion so it is not that difficult to replace the rails, as I anticipated this might be necessary. I can’t see any ultimate solution, other than using metal. Aluminium of the scale required however would be expensive, and steel is very heavy. I did try just surfacing the rails with aluminium, but that caused a different problem: water got capillarised in through the narrow gap. I intend to replace with new timber, but I may try varnishing with yacht varnish or something hard and hard-wearing, in an attempt to increase durability. But in the end none of these solutions last for ever, and 12 years is not too bad.