Motivated by Dennis's thread but not having the equipment to try building one, I stuck to the old Hartmann design and spent a lazy cloudy Saturday afternoon making the attached from 4mm MDF for my 80ED.
The holes are round it's just shadows causing the distortion
Put some line across each hole, this will increase the accuracy humangulsly.
Then you use the diffraction spikes to focus.
Theres plenty of info out there on it.
