5-litre container lid opener for 8-lipped caps

It's a remix of the original, basically the same design but for 8-lipped caps.

I've included the source freecad file which has parameters for the amount of lips and diameters and thickness, so it will be easier to adjust for others than it was for me.

Slic3r project file also included.

Changelog:
- mk1: initial upload
- mk2: better fitting ID, optimized fillets
- mk3: OD larger for strength, added hole