How to make a raft buoyant?

You'll need BIG logs, or lots of small ones... water displacement equivalent to your weight is the key. If you can't find jungle vines to tie them together, you're going to have to learn some wood working skills... like mortise and tenon joints... OR if it doesn't need to specifically be a "raft" you could hollow out a log like the native americans did to create a canoe... just depends on how much time you want to spend. Best of luck!