Vorpal Mag 18 Round Back Plate - With Thin Membrane Window

This is a quick & dirty remix of the 18-dart [Vorpal Mag](https://www.thingiverse.com/thing:3097776) back plate with window -- all I added was a thin 0.40 mm membrane to cover the otherwise open window area. Inspired by the thin translucent follower channel on the [KaThanha mag](https://www.thingiverse.com/thing:3069519).

REQUIREMENT: This means you should print this part using a transparent/translucent filament to allow you to see through the back plate and gauge the remaining dart count in