I actually went through a similar thing when the spring steel broke on the flap for my GMC. I already had a variety of neodymium magnets around and found that a single 3/4" x 1/16" N48 disc worked perfectly. Just enough that it never opens on accident yet is very easy to open when needed. Also, I put mine on the flap itself so it's never even noticeable.
Etching PCBs with Press'n'Peel