How 'bout we put a sight glass on one of these RMND units?

While some of us are changing every thing around anyway . . . has anyone put or contemplated putting a sight glass on these? One in the door would be pretty trick (keep the newbies from opening the door all the time) but probably not worth the loss in insulation value there. But what about some where along where the HX tubes turn 90 deg and go vertical?

Jimbo