My advice is a set up like above if you're only doing one. If you're dead set on a rack you'd have to use a space heater or something to warm the area.
Someone else may chime in, but options are pretty limited to raise ambient temps in a rack or a tub. The way I have pictured above gives me perfect temperatures.
A lot of people have a whole room they'll heat, or back in the day when I had a rack set up, I had the rack in the closet, heat tape on a thermostat for the hot spot and a space heater on a thermostat keeping the air temps in the closet at 80F. I didn't have the option of doing a room, so that's what worked for me.
Best of luck.
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