Actually....
You can fit a cover over the stagmag. The magnetic poles have the same diameter as the slugs and you can have slugs through the covers. The bobbin is a standard bobbin after all. And if you want a closed cover, no problem. Just push the polepieces down. The wire is wound on a bobbin, not on top of the magnets. I know because I took a few apart, and did all of the above myself.