Nice Job, what a good use of your skills to overcome a need.
Quite a few years ago we manufactured a card access controller, that had the same problem, of excluding moisture.
We mounted the display behind a piece of lexan (clear plastic) that was sealed against the box.
The display was mounted to the plastic (which was oversize), and the plastic glued to the front panel using RTV....no mounting holes.
We used either a rubber keypad, or a vandal resistant version, purchased through RS Components, which mounted on the outside, and was sealed at the top and side (so any moisture drained out).
The only problem we had was with the vandal keypad (metal keys) when it was cold. It seems that the keypad off resistance dropped, and the interface IC detected it as an ON, even though it was no where near the On detect resistance.
(We fixed that with pullup resistors).
Any keypad that uses a continuous rubber membrane should be moisture proof from the front.
Its not always moisture getting in thats the problem, its making sure it can drain out.
The commercial access units often have a cover, which stops most water, ... and vandals.
Good luck
Mark