I built my current 10ft x 10ft roll-off roof observatory about 12 years ago. This was built from the ground up and not based on an existing shed.
For the runners I used angle iron from a local agricultural blacksmith (I live in a rural village, so easy to obtain). I suggest angle rather than channel to avoid side pinching. For the wheels I used 75mm Non Marking Wheels from B&Q, 4 on each side. These are plastic wheels with a hard rubber tyre and 8mm central hole. I though these might fail under regular use so I bought some spares but have never needed to replace them. Reference numbers on the package (whatever these might mean) are H29566 and 9436701. The biggest problem was transporting 20 foot lengths of angle home on a car roof rack. I’ve been very pleased with the observatory and would do the same again.
Good luck, Stewart