I wanted to have a thermometer that went down to the middle of my mash and stayed there so I could monitor the temperature. Even the long dial ones don’t reach down very far, and it’s hard to find something to fasten them to when you are using a keggle with BIAB. They are also harder to see incremental changes in.
So I bought a digital thermometer with a probe. The first time I used it, the readings were very wacko during the mash. Only then did I read the instructions and see that you weren’t supposed to immerse it beyond the solid metal probe part. So I took a length of seal-a-meal vacuum sealer bag material. I cut it lengthwise, about two feet long and three inches wide. One side was already sealed. To seal the other long side and the bottom, I used my iron and a piece of parchment paper to protect the iron and ironing surface from getting melted plastic on it. When I thought I had a good weld, I filled it with water to check for leaks, then let the inside dry out thoroughly.
I use a long spoon to push it down into place, and it stays there. Works quite well!