This is a test sheet of T-nuts.Some are of the original type posted by kludgineer and some have been elongated for greater support when attaching things to the frame.The main difference that should help with MendelMAX assembly is a dual t-Nut spaced at 20mm, most things on the frame are spaced for this to assist. Print out a sheet and experiment, you should be able to fit a M4 nut into the recess after drilling out the hole and use it as a T-nut.