jooohn cutting off a corner here and their and such is not going to solver you problem.

Trying to make a case that is not designed for your motherboard will cost more than a case.

Your stand offs must be correct in all respects and they also must positions the in/out panel correctly.

Just do your homework for a case that is designed for the motherboard of your choice. If your choice is a extended ATX motherboard then find a case that is designed for that type of motherboard.

It's no more complicated than that.