The stock Fender back also keeps young fingers out of the hot tubes, and are less than optimal. Head cabinets of limited height and the chassis on the top can have particular problems. Not a Princeton so much, but a SuperTwin chassis in a head cabinet.
A much more effective Fender cabinet back would have an "L" shape, forcing the incoming cool air farther inside the cabinet. And a taller cabinet improves the chimney effect.
Also, tubes cool a lot via radiation, not just convection (and conduction only via the pins). Realize that there is NO convection thru the vacuum, and ALL the heat from the plate etc. is initially conducted to the glass via radiation, and a considerable amount is radiated thru the glass (depending on the glass, quartz content etc. of course) or turns to lower frequency heat at the glass and re-radiates from the outside. So it's useful to imagine your output tubes were light bulbs and to allow them to "shine" outside of the cabinet, instead of "shining" right at the cabinet back.
I love the metal reflectors that Reeves put behind the power tubes in his Sound City and HiWatt amp designs. The problem then is that though they reflect the infra red out of the cabinet, protect the can caps and transformer, the reflector also blocks convection cooling somewhat. But it's something to think about.
If you do use vent holes in the front panel and cover them with grille cloth, make sure you paint the panel black first. I made my panel white then put on grille cloth, and the dark black holes behind are too obvious.
