[glend (Glen)]

Hi Allan, I actually bought most of my roof roller equipment from Bunnings. I used simple galvanised steel angle iron, the reason being your wheels run on one surface and are contained by the outside face. Sure you can use C channel, but there is no need (imho) for an internal channel face if you have installed the L track properly. A wheel cannot leave its running surface to fall inside if the external side of the L channel contains it on its running surface on borh sides of the roof. I used simple nylon flat bottom caster wheels, and I think there are photos in my old obs build thread here. I have track stops made out of wood blocks at the end of my tracks.
For roof hold downs I use simple rated tie down straps at each corner which are looped through my corner braces. I highly recommend 45 degree angle corner braces at the top of each wall joint.
My obs has come through sustained winds of over 100kms in the big East coast low storm we had a couple years ago, no leaks, no damage.
For gap sealing around the roof to wall joint I simply used plastic brick damp course material, which is sold by the roll at Bunnings. It's black, textured, and easily stapled to the side of the roof as a track curtain. I covered the staples and top of the plastic curtain with a timber side curtain on the roof, so only about 10mm of the track curtain shows. It keeps put rain and wind and cannot blow upward because the timber side curtain contains it. I also used the same material to close the front gap where the roof slides back, I chose to use it instead of a solid flat type door. It bends easily over the track when moving back and forth.
This is all had to explain in just words, the photos in my archived thread will help, but your welcome to drop by and inspect my obs and get some ideas. There are many ways to arrive at the same function. Just PM me if you want to inspect, i am in Brisbane till the middle of next week but it's available after that. Cheers