When using a cooler I use one with a drain plug (120 quarts cooler can hold (8) 6 quarts tubs since you want something small), I pass all my wires through the drain plug and place some playdoh in the hole which I cover with tape afterward. (Better alternative than foam if you need to change a cable). If I use something without hole I drill one and do the same.
Fans are not always necessary in small coolers (I have tried with and without with no difference)
When it comes to probe I always have a dummy tub in my incubator (set like a real tub no water just perlite), I tape the probe inside the lid and cover the hole I drilled with tape. The tub is placed exactly in the middle of the incubator.
I have done that for years and it never failed me but your dummy tub must be in the middle of the incubator (same principal that when you put a probe in the middle of a rack.). This way I don's have to worry about my probe getting lose if attached to the heat source.
And yes there are diy in the diy section for various size incubators, it's not that hard the most important to keep in mind is to get a RELIABLE thermostat.