So, at our makerspace's new building we found we had puddles which weren't good, and happened to have some 4" flex pipe, but the drain is a 4" by 5.5" pipe coming down.

So here's an adapter, which should be customizable either for Round to Rectangle or vice versa. Set the circular_use_id (Inner Diamater) to choose which to use. (Rectangular, just enter the dimensions (-2*wall thickness) if you want it to go on the inside.)

Anyway, I hope this is useful to someone, and to us (as it hasn't yet been printed.)

Edit: 2024 March 5: Added -3 which corrects the centering if you adjust the thickness (It wasn't correct with the 2mm thickness, but I never noticed. Ministry42 pointed it out.