eman, here's something I'd done and you may give it a look:
Instead of light-weight rubber, you could try some thing heavier.
Make sure you try out the proper weigh distribution first, on the outside, by whatever method convenient. After you find your preferred distribution and balance, mark the positions of the weights and "mimic that" into the inside of the racket.
If you really need something heavy for cheap, try tyre-balancing weights. I usually get them for free and they come with their weights marked. You can then hammer them down as flat as you want.
Hope this helps.