I have made one myself: basically an aluminium corner cut to fit the fin, "filled" with one piece of wood cut to hold the fin (plywood as thick as the fin: 5mm I think). works quite well. I could thus use a proteck small future fin for my daughter in the US box.
The trick was to put the aluminium flank on one side only, there is no room between the fin and the box to have enough material on both sides I guess.
more pics at:
www.gong-galaxy.com/forum/viewtopic.php?p=50874
