I had completely forgotten about editting Custom.py!
I think I didn't realise for a long time that I didn't need to do it, or to use BCMP to install ships! We're talking 2 years of me wasting time before I forgot to do that one day, and lo and behold, the ship worked, right off the bat!
I thought "WTF!? Some update to foundation or something?!", hehehe, so stoopid!