I would not recommend aluminum in the fermenter. It's fine for making wort, boiling etc. But during fermentation the pH drops and the wort becomes more acidic. Aluminum and acid are not friends and I'm pretty confident you'll end up tasting the aluminum.
Most small hardware stores (at least in my area) have a little rack of various SS shapes. They almost always have SS tubing and 12" of 3/8 is probably less than $5 You can fold and crimp the end but make sure you clean it really well before you use it again or better yet bake it each time.
Aluminum thermowell?
Linear Mode
