Yeah, I'd say 20 minutes working time at 75 degrees is pretty right on. It starts to get tacky at that point and the flowability decreases a lot. It's plenty of time to get a nice lam finished.
Like LBCrew did, I made a hotbox and put the boards in there to speed up the cure after each lamination. It's awesome. If you need a telescoping blank box to make a hotbox let me know.

I just dropped some ProBoxes in with Quik Kick (Also termed Kwik Kick) and they did not float. I only fill about 1/4" routed hole depth with resin and drop the boxes in, then finish around the outside with a dixie cup...