According to Andreas Moritz in his book, you can use magnesium citrate powder in place of the epson salts. Same amount and timing. It's not tasty, but a big improvement and easier (for me) to get down. I've had a number of successful flushes using the mag citrate, so I can vouch for it's effectiveness.
I use a product from "NOW Foods" - may have gotten it on amazon, but should be easy enough to find.