Rack temps

So I have a simple rack system with 4 snakes in 28 quart tubs, separately of course. I run 3 inch flex watt on the back bottom controlled with a zilla 500 watt thermostat set at 92°. I test the temps weekly with my temp gun and I have a secondary electric thermometer always on. My question is, how do you actually test the surface temps inside the tub when it's pushed closed? I mean, it's not like I can fit my arm back there.

Thanks in advance

At the place where I work, they have their temp. probe on the heat tape in an empty spot, where there is no tub.

Edit: There actually is a tub, whoops, the probe is in the empty tub.

Why do you need to do that (measure when its closed)? Just pull the tub out as far as you need to, clear the substrate out of the way, and use the temp gun on the spot(s) on the tub that was over the hot spot. It won't cool down nearly fast enough for you to be concerned about that.