Hmmm.. I'm not sure, I think the back one is much bigger than neccesary and the technique focusses on the thoracic spine rather than the neck. Also as the sequence is 3mins cold, 1min hot, 1min cold, 1min hot, 1min cold, 3mins hot, you need to be able to change over quickly so the harness would be a bit awkward .
I use two rectangular gel packs (280 x 130mm) side by side as they match the size of the hot water bottle I'm using for the hot part. I put then on a towel on the floor and lie on them alternately.
Have you got the book? It's probably worth it if you were thinking of trying it out. I'm happy to discuss how my osteo recommends doing things but I don't think I've got the stamina to post the exercises in detail (typings pretty tricky for me).