You dont want to get nitric too hot depending on concentration, you can evaporate the acid, instead of having it work for you, if dilute mostly water vapors off, but this can concentrate the solution so that you are closer to concentrated azeotropic solution where your acid will boil off, warming (or just more time), should be all that you need to increase the nitrics dissolving power, I personally would not use the crock pot for nitric, as I prefer glass, the crock pot has a layer of glazeing that will break down with strong acids over time (pitting), I do not think pressure from lid would be an issue, the gas would escape, how about a coffee maker (or warmer burner) and old coffee pot, they just get warm and if you want more control a triac lamp dimmer can be added to make it more adjustable.
nitric when disolving metal can also create its own heat from the reaction, keep that in mind, also nitric can concentrate to a point with heat that it will start to foam and can boil over if unattended, keep that in there too, the red gas is terribly corrosive and dangerous, use fume hood , outdoors,a scrubber, or condenser, these fumes will ruin a shed for any other purpose, and could possibly make it more prone to being a fire hazard.
