car tent question

I am working on a tent to fit the back of my subi. I want to be able to sleep in the car at ski area parking lots. My question is should I use an outdoor fabric like Sunbrella or should I go with a Silnylon? Sunbrella takes serious weather punishment as shown by its extensive use on boats. The potential problems are that it is bulky, heavy, and I fear snow will stick to it. Silnylon is not at durable but I suspect it will not hold snow as much, it is lighter and less bulky. It will have to be inside the car during the day and so it would be helpful to reduce the amount of snow stuck to the tent when it is put away.

Any feedback as to the performance of these fabrics will be appreciated.

What are you trying to make? A tent to extend off the back of the car or just like a sock with a window to put over the open back door?

I've been considering making the latter out of an old tent for trailhead crashing. The main detail that i need to get hammered down is making it bug tight for summer nights. I haven't put much thought into it yet though.

I am working on a "Sock over the hole" design. I am to tall to sleep on the bed of the rig, so I need to extend the bed. I have a platform made and now need to manufacture the sock to keep the inside of the rig from drifting over in the night.

There's a tent fir sale for 35 bucks in the personal yardsale section that you could probably harvest all the material you could want from including doors and screens. Might be easier and cheaper than starting from scratch (at least that's what my busch league approach would be).