Use Titebond. I've glued far too many MDF boxes together, and know what NOT to use - gorrilla works, but it'll swell and expand past your edges. Once it dries up, it's a pain to remove all the bubbles etc.
I'm really interested in how these MDF forms will work out for you guys, my experience with it leads me to believe it might work for a couple bows, but it won't last. Good maybe for prototyping a new design?
Also, are you guys using the 3/4 or the 1/2" MDF to make your forms? Just curious...
Another option could be HDF, but it's not as easy to find, but much more stable.