I have to agree with Boydie on this one.
I've been there and tried that myself and was never able to achieve anything really satisfying. I certainly can't say that it can't be done, but I gave up trying. I broke down and bought a cheap bass and I'm really glad I did. It's fun to play a bass.
If you can't buy a bass, and don't care for synthetic bass, you might try a collaboration? Someone might be willing to play the bass line for you. Especially if you already have the bass line you want written out in either tab or music.