Or you use a regular license, but publish the terms openly.
Ie. "Anyone who uses this printer design, or one based from it, must pay $3 to us per printer sold. We will give this permission to anyone who asks. For anyone who contributes back to the design of our printers, we will consider reducing this figure to $0"
Ie. "Anyone who uses this printer design, or one based from it, must pay $3 to us per printer sold. We will give this permission to anyone who asks. For anyone who contributes back to the design of our printers, we will consider reducing this figure to $0"