I have a similar four sided mold with screws on all corners. Before putting the screws in, I stapled heavy plastic to the INSIDE of the four outside pieces. I use 2 squares of styrofoam for the inner form and just wrap it in plastic wrap. Don't tape them together. One piece would be difficult to remove, but using two, and removing one at a time makes it much easier. I have used the same mold for at least 10 troughs. Hope I made this clear enough. Let us know how it works for you.

Regular motor oil painted on the wood will act as a release agent.

Sue

Hi,
Clean up all parts of your mold. And make sure each of the 9cubes is dry. Then paint the entire mold with a durable outdoor latex paint (your choice of colors), or a marine paint. Once all of that is dry you can use your oil painted on with a cheap brush to get all the area covered.
I suspect that the wood is leaching just enough water to swell after the tufa is on it. Plus each time you use your form small scratches are created in the surface which will then allow your mix to grab the form the next time.
Dena

Thanks everyone for the helpful suggestions. I think I will try them all and see how they work out. I'll let you know how it turns out.....probably in a few weeks.

SereneSeen