What to make roll-cage out of?

You could also bend metal rods(Copper, etc) and solder them together.

Really depends with which material you are more comfortable.

Use some plastic rod from Evergreen.The seat could probably be found in one of the tuner kits out right now.The Evergreen rods can be had for $2 a pack.

since the methods of cage i was going to say have already been said, there is a really similar seat in the Revell C5-R Corvette. its the one that Dale Earnhardt drove at the Rolex 24. if your interested, i have a spare one handy i could trade. all you would have to do is flock it and glue it in. ive been thinking of building this myself. good luck. later.
Jon

I would either use Styrene tubing or Copper. The Styrene of course being on the least expensive side.

I've made many roll cages and I use either steel or copper rod or styrene tubing of the appropriate diameter . I don't bother with solder on the metal and copper, super glue works just fine. One of the big problems is on the real car the roll cage is right up in the roof. On a model the windows are generally a one piece clear piece. This means the roll cage won't fit up into the roof as well as on the real car you might have to play around with it to fit nicely. This is where styrene is preferrable as you can trim it better.

if your on a budget, use a piece of the spure. you may even be able to incorperate i piece with a 90 degree bent already molded.