A magic portal designed for our 18mm scale Wayfarer miniatures (same models we use for Pocket-Tactics). If you want to use it with 28mm scale models, I'd scale the designs up by 166.6%, though it's already pretty large, so it could really work without scaling as well.
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Printed mine at a .2 layer height with 15% infill. The portal itself was printed in Hatchbox translucent black ABS.