Windows 7 Pro 64bit on Late 2015 iMac via bootcamp - needs windows 10!

I have a Late 21 inch 2015 iMac and would like to install windows 7 Pro on it via bootcamp. (I know windows 7 but it's to test out mods to windows 7 for a $100k piece of scientific equipment that still runs windows 7 and it not upgradable - also on a private network). Bootcamp runs fine but giving it an iso file it complains the it can't only install window 10 64 bit. Any workarounds? I don't want windows bloody 10 I need 7, 7 pro :) Machine works fine lots of ram and hd space.