The specs for 2020 aluminum profile is here. https://8020.net/20-2020-black-fb.html This site even has the cad profile files for download.
I have noticed that some people have suggested using V-Slot from open builds... however, V-Slot has a V for the V groove wheels which this design is not using so in my opinion there is absolutely no reason to pay a lot more for V-Slot when your not even using V groove wheels.
I have also noticed that some people have suggested using 2040 T-Slot and the link above has a search and you can find the profile for 2040 T-Slot. I agree that T-Slot could be better; however, I would build first with 2020 T-Slot to see for yourself and then report your findings after upgrading to 2040 T-Slot. My guess is that you will find that 2020 T-Slot is enough if you use the recommend corner brackets... if your planning on using 3D printed corner brackets... then it might be necessary to use 2040 T-Slot.