I'd go with a stack-on also if I were in an apartment and just use trigger and cable locks on them to meet the MA requirements and not sweat the outer lock. As for adding hangers for handguns, think wood and dowels. Drilling through thin metal and into a wood backer doesn't take much by way of tools.
I don't use safes. I just build things like furniture or compartments into the walls of which I need to start a new one. I've been on a small buying spree this year and need to build for ten new ones that are locked in the above mentioned pelican cases. Anyway, what I meant with the above as a sample for dowels with a wood backer.
View attachment 761906
(If interested, this is the inside of a chest of draws. It's 5' tall, 29" wide and 17" deep. Fairly small but it holds 13 rifles, 8 pistols on the inside with an an O/U two barrel shotgun set and 2 more rifles on the swing out door along with about 6 fix blade hunting style knives and various accessories. I could add more but then accessing them wouldn't be as easy. Best part is, reconfiguring what I do is easy as it's just wood and screws.)