Rectangle base pack (magnet stand)

I have tweaked my original round bases to make them easier to use.
I also made [round](https://www.thingiverse.com/thing:4927789) (cavalry) and [oblong](https://www.thingiverse.com/thing:4927797) (cavalry) bases.

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

File name explained:
* **Rct**: rectangular (general base shape)
* **45x90**: base dimentions (in mm)

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