OP
P3-12's on 1K?
Custom sealed?.. lol
Why not step up and you listed some serious space available.
P3's like anything from simple ported to a higher-order enclosure.
Sealed?.. at least do a 4th in there if you got the space to do so.
Jmo
Anything will be an improvement over sealed... most "custom" boxes are small sealed in my exp anyways.
A low tuned ported enclosure may be the ticket if something crazy seems undoable to you Op.
P3's are versatile, but in small sealed they lack lows imo.