Then, the approach should be more like with one time donations.
For example:
Donate 5$, and receive a 3D mode of a fighter/freighter size ship. (You choose ofcrs). AS GIFT!
Donate 10$, and receive gunboat/transport 3D model.
Donate 15& Cruiser and so on.
What is important IMO- you wont be making direct sale of those figures. Which means, you could print even vanilla models (I might consider buying one). This wont be considered as commercial activity.
Also, at the beginning, it should be organised here on Disco GC forum for members and players, before going to public trading platform. Or might be even stay here.
So in simple- Donate X amount of $ or Euros, and receive fancy 3d modelled ship.