Oblong base pack (magnet stand)

To complement my [round](https://www.thingiverse.com/thing:4927789/) and [rectangular](https://www.thingiverse.com/thing:4927800), I made oblong bases that can be used for cavalry/bikes miniatures.

I am using 10x5x1mm parallelepiped magnets that I buy from [Supermagnete](https://www.supermagnete.fr/eng/block-magnets-neodymium/block-magnet-10mm-5mm-1mm_Q-10-05-01-G)

Improvements:

Stand for magnet underneath the base:
* More adhesion surface: the stand is now fully filed.
* Magnet easier to place: the stand is now bordered with a rim on two sides

More bases will be added as I create new ones. Stay tuned (or watch this thing)