Someone here from Florida mentioned using a cheap pool raft, and I've used those before, years ago. I have one like he recommended, got it at Kmart for $4, but I don't think they carried them this year. It's called a "Clearwater Easy Mat" if you can find one. They're not really light, but they are comfortable and pretty durable.

A good, inexpensive, lightweight sleeping pad is not easy to find or make, as far as I know. If there are any trees where you'll be camping you can always try a hammock. There are some inexpensive options to try those out, and lot's of DYI options.

I use the single layer bubble foil insulation BZH mentioned for insulation, it's too thin for a pad. They make a double layer, and I've seen some with BIG bubbles that might work as a pad, it'd be pretty bulky, but it doesn't weigh much. I'd give it a test if I found a big enough piece