I have been giving a lot of thought to sealing the electromagnets. It will be difficult to do a seal with fasteners that will keep the leak rate low enough.
It may be necessary to weld a cap over the seams or do some other kind of welding or perhaps silver solder if an appropriate metal is used for the coils. Electron beam or laser welding may be an option.
The coil design should be well tested before we seal them to make sure that unbuttoning them is not a regular occurrence.