Since I am printing the frame, I decided to split it in two logical parts: the top and the bottom that screw in together using standard M3 screws.
A while later, I had the top part.
During initial testing I found that the 10×10 dimension for the tubes is too weak, I will have to redesign this to use 20×20 tubes. I also found that the printing material should not be PETG, because of the nature of that material. PETG has a tendency to flex, and I want the frame to be really stiff in order not to interfere with accuracy of the head. So I will be reworking the frame in ASA filament, which I have had good luck printing in the past.