For one-off events, we have a ghetto version that works pretty well...

Once stacked a Hobie 16 on top of a Hobie 20, using 2x4s wrapped in bath towels as spacers (placed directly on top of the H20 hulls), from eastern MA to Madcatter (300+ miles each way). Used ratchet straps, tied the 16 directly to the trailer (used extra towels to protect straps from chafing the H20), no issues at all.

Mike