I wonder if it would help putting a hand warmer on the eyepiece by using an elastic band to secure it on the eyepiece, (the side of course), ?? Hmmmmm....Has anybody tried this?
The chemical hand warmers do their best when in an enclosed space that will trap the heat such as a coat pocket or in an eyepiece case. Out in the open they don't hold up to the task very well. Having said that, I have used them inside a refractor dew shield by laying them along the lower edge and they did the job for me in a pinch. They will stop dew, they won't clear it once it starts. I have hand warmers under the eyepieces of a pair of binos and they were completely ineffective as the eyelenses fogged up as soon as I tried to look through them.
There are supplies of reuseable chemical heaters that put out a lot more heat than the little hand warmers. You can find some in a camping section of a sports store or in the baby section of some department stores. Boil the heaters to charge them. Shake them up to activate. You can get a wrap around model that is made to warm up baby bottles. (Should fit a 17mm T4 rather well.)