[Marsupial]

I have a 47-1230 too.
The 5Y3 DO run burning hot. (BrendaAnnD: you WILL burn yourself if you try to unsocket the 5Y3 without waiting several minutes - we're talking seriously hot here)
The cabinet becomes warm (not hot)

The radio was designed with that in mind: the socket will withstand the heat. Its been doing it good for 70 years already.
Heat won't get inside the radio, by nature. Its plain physics. Heat rises. Goes into the big area, and into your cabinet. As Arran pointed out, the tube's pins won't even get warm.
If you are mainly scared for the chassis.... don't. Those things are heavy duty.

If you are mainly scared for the cabinet... I could understand the concern, but I don't think there's a problem there either. The top of the cabinet is where all the 5Y3 & 6V6s heat goes, you can feel it. It doesn't seem normal - I even asked if it was "ok" on these phorums when I started to work on my 1230...

but, When I refinished my cabinet, the top of the cabinet was the single place with the less damage to its finish. The front of the cabinet had chips, the sides had marks, scuffs, and what I assume was UV damage from 70 years of sun exposure, the round areas had lost lacquer in a not so good looking manner... but the top of the cabinet was pretty much intact.
I don't think that mild heat is a big source of concern over lacquer, compared to scuffing or permanent sun exposure.

Now, keeping heat away can't do much harm either. I didn't think about doing that (being told that this heat was normal...)
I am curious as how you did it.
If I was to do it, I'd get a 80mm 6VAC fan and connect it directly to the chassis trough the phono lamp wire. That might have to do with the fact I have no phono lamp and these wires are loose on my chassis...