I had bought some flexible magnetic tape from the hardware store a few months ago for a project around the house. It was a large roll, so I had a lot left over. Tonight I put some to use on a DIY magnetic fly box:
1. Roll of magnetic vinyl tape - I'm using the leftover 3M
2. Empty Altoids-type mint tin - I'm using L.L. Bean Fisher-Mints container. (I'm sure if you're into dipping tobacco, an empty tin of Skoal, Copenhagen, etc...would work too)
3. Cut size appropriate strip of tape and line the bottom of the tin. In this case I actually had to cut a strip and a half as the tin was wider than the tape.
4. Drop in some flies. In this case my typical tenkara arsenal of Beadhead Prince Nymphs & CDC Elk Hair Caddis.
I've shaken it, tipped it upside down, they don't seem to fall out. May not be great for bulky streamers or dries, but for these size #14 nymphs (& I'd imagine smaller flies), it seems to work pretty well.