Product review (4 stars): Easy way to ensure a freezer food hasn't thawed, plus Velcro strip keeps the "meter" securely  in one spot in the freezer
Calling this a "meter" seems a tad misleading. In my mind it is more of an informer. Does it work any differently from putting a coin on top of frozen water a small container in your freezer? No, in fact it works exactly like the DIY solution. The difference for me is it has a Velcro strip attached so I can fasten it to one of the freezer shelves. This solves the problem I had with the DIY solution: after a while I could never find the danged container. With 3 freezers, two of them deep chests, I was always covering up or knocking around my other containers. With the Velcro attached to a shelf, it never gets lost.

Could I fabricate something similar? Sure, but for the price, I'd rather spend my time doing other things. After using one, I bought two more, one for each freezer.