The foam thing looks like a great option.
I bet the 2x4 idea started way back when there were no cats but H14s and H16s. You know, before foam was invented...
I know for sure that I was using it before using the internet.

Mike
You're exactly right. Nailing a couple of sets of small "risers" to the bottom of the forward 2x4 was necessary to compensate for the beam curvature. Rear cross-2x4 can be a single piece.
Slap some carpet on it and you're done. (note - bottom H16 facing forward and top H16 bows to rear, or you're really popping-a-wheelie!)
I did the packing foam and swim noodles thing to take an H17 & TheMightyHobie18 to NAC's last spring. Cheap and functional.
Found that I needed to tie from the bow tangs of the H17 to the mast stand.
In big cross wind, top boat would eventually work itself off center (no matter how well was tied to bottom boat)