Well I can't give you any concrete advice as I have never owned a tank similar to the one you have there, and anything that resembles a heat rock I would never suggest. You could cut the bottom out and replace it with plastic or glass to allow heat transfer, but I really don't know how else to heat something like that.

And on another note, if you are practicing something generally frowned upon such as keeping multiple snakes and you mention that in your post, you should expect people to point out the danger in that practice. If not to warm you, then to warn others that may not know it is something that should be avoided. You could have simply said "I have a new boa, should I try and use this enclosure or buy a new one."