Thanks for the comments folks.

Unfortunately I have to repair one of my mobies. It broke when I allowed it to snap after removing the hatch. I'm not sure off hand what diameter of bungee I used, but it has a lot of force - I think it's 1/4". I didn't reinforce the mobies with fiberglass & built them out of an unknown 3/16" plywood that looks like mahogany & was salvaged from an equipment shipping crate. The space that I was to fit the bungee through was a bit tight, so I drilled it to remove a bit of wood and allow the bungee to be pulled through when stretched. I think I removed a bit too much wood on the one that broke. My advice is to over-build your mobies (or use materials you are certain of the strength of & don't go removing a portion of what strength it has).

Cheers,
Bryan