The Manito line looks interesting. I like the concept of the floating hardware. A project that I have had in my mind for sometime now involves floating hardware as well but I just don't have the time or money right now to get the laboratory going!
To answer your question about whether these shells could crack? Wood is a natural product, therefore as a building material, whether it's musical instruments or ships, there are almost always certain variables that are unknown. A Donald Rumsfeld quote comes to mind here:
"There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know."

This quote is quite apt in relation to woodworking too!
Slathering on a thick layer of varnish will prolong the life of the shell but if there are any stresses in the wood (working the wood with too high a moisture content, improper drying/curing, embedded objects like nails or lead shot that are not visible to the eye), in time cracks could form. If the shell is coated in varnish it would make it more difficult to repair because all that polyurethane would have to be sanded off!
Moperc drums are slowly getting more widely known. Michel had some pretty strong endorsements out there. I've played them and I must say that I really like them a lot. If I were shopping for a set now I would certainly buy Moperc. Not to mention, Canadian made in
La Belle Provençe! And his shop is only a few hours' drive from where I live now so I could just drive over and pick them up myself.
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