YouTube member Sunny01 shares a a simple DIY roller bearing slider most people can build out. A few T-shaped joining brackets on each side, some carefully spaced bearings on top and bottom allow the carrier to ride over the PVC pipes. If PVC isn't your thing, there's no reason you can't redesign around some lightweight copper piping. [Thanks Sunny]. Judging from the build and how it maintains itself on the rails, you'll need a minimum of 12 skate bearings. Normally these bearings come in packs of 8.