you can tape your aquarium probe to the outside of the lbk below the water line, then tape a small piece of reflectix insulation over that and you will get a ballpark idea of wort temp. those thermometers (the yellow one above) are not 100% accurate but close enough. I use them. my lazy man's guide for temp control is if my cooler ambient air temp stays around 62f then my wort temp should be ok for most ales. 62f is good on its own. peak fermentation would mean wort would be around 72 max probably...which is a little high but still ok for most ales. . at least for me. ive only had apple esters on one or two batches. one was because I used us04 in a stout and let temps get way too high. imo stouts and apple esters are a major no no. if I wanted fruit in my beer I would dump a can of it into my glass.
here's a thought. if you were confident about sanitation you could probably soak your probe and about a foot of lead wire in starsan for a bit... then run the probe directly into your lbk but how would you screw the lid down I wonder? or you could take a meat thermometer , sanitize the probe end and poke it through your lid into the wort... but then you would have a hole in the lid. I don't worry about it. I just shoot for ambient of 62f.