Echo that the 501st does not accept CUSTOM Mando's. There are a few Mando's that are canon and accepted, but not custom. As for the back of the helmet, customization would look best if it fits the type of mando you are building. Does he/she have a special environment he specializes in or a special bounty he likes. Is your mando a technology splicer, medic, heavy weapons operator, sniper, etc. Thinking about that might help you decide what would be useful. If you need any Mandalorian Mercs info, I am an official Mando and id be happy to help