Lots of different options ... just substitute a copper cap for the candles ...
Or add a riser underneath to get it more into the open air. It do get a bit crowded on a Dynaco chassis, especially with the competition of all the other heat sources.
Not sure if the vent holes around the socket could be done with the stock chassis - I do know they help, but you'd also have to vent the bottom plate to allow good air flow for a chimney effect. Air's gotta come from somewhere.
Or build a custom chassis ... lot more room for the big boys to breathe. As mentioned, I didn't roll with a copper cap for long, but had no problems when I did.
Or add a fan? Nice flat unit on top of the PT would be easy enough to do and blend in fairly well. Wouldn't even have to be permanent. Bend a plate to use the PT's corner screws, attach the fan to that with stand offs, and power it up with a USB wall wart. Some clever bending on the plate, and you could even direct the air intake for max effect at the rectifier. I've seen fans mounted to the back where they're not seen, and once again, a baffle plate could be use to cool both the big iron AND the rectifier.
Water cooling is an option ... make it adjustable for each valve ...
Couple things to keep in mind before bad mouthing the design of the copper cap too much ... they're designed for musicians, for musicians, and we're not using them for their intended purpose. Also, a typical instrument amp will have the rectifier mounted horizontally which would induce more air flow across the entire shield, and have more air space below to allow improved chimney effect. So there ... ;-}