If you're building a custom Mando, any 1:1 resin helmet is fine. Be it a generic Mando, like Gypsyboy makes or, a Boba Fett Mystery helmet from Sgt. Fang (there's one on ebay right now if the Sarge doesn't have any available). If you're building a screen accurate Boba or Jango, you need to look at the likes of Boba Maker, Asok's LFL Jango the BKBT helmet etc.
But, for a custom Mando, you can save some money and skip those, with the exception of the Asok which is very affordable and availble in the Cargo Hold here or the Trading Station at Mando Mercs.
My biggest pet peeve with custom Mando's are tic tac headed Mando's from using too small of a helmet, chins hanging out the bottom of a too small helmet (even when covered with a balaclava) and bobblehead helmet from the helmet being to larg (very uncommon except with children and in that case I accept it do to what's out there and their size). In fact we've decided at Mercs that we are now going to seriously crack down on helmet size as it's applicable to proper armor sizeing and placement. Head look like a tic tac, we'll deny an application and request they get a wider helmet. CHin hanging out the botom? We'll deny an app and request they get a larger helmet.
In the case of adults, this can be fix on almost everyone by forgoing the Rubies helmets altogether. If you're medium framed a Don Post 95 should be fine. If you're larger framed, nothing smaller than a 1:1 will suffice.
I would highly recommend signing onto Mando Mercs,
www.mandalorianmercs.com , if you haven't already and check out this thread...
http://mercs.firespray.net/forum/index.php?topic=13291.0
This thread was written specifically to assist new and existing builders with properly sizing their helmets.