S25,
1/2 ply on a card table may have a tendency to sag in the middle. You would have to give it a try. If it sags, you will not be able to build a straight airplane on it. Probably depends upon how rigid the top of the card table happens to be.

I used to use a hollow core door for a building surface. They are light and rigid and inexpensive. You have to check them out and find the truest, that is to say flattest, one you are able to find. Any lack of flatness in the surface will be built into the wings... and everything else, but it will show up most in the wings. Take a straight, straightedge (they are not all straight) or perhaps a long, aluminum level (they are pretty straight) to the building supply store to help you find a nice true door.

Good luck with your build, it will be fun.