Thought that after years of reading everyone’s great ideas and feedback I ought to throw out an alternative I use, which is an old Chest Freezer as a hot box and that I get very good results with on all Juglans. I’m in England and summer bud grafting isn’t reliable here so winter grafting of Juglans/Carya and other hardwoods is the only reliable solution.
I’ve tried hot pipe systems like those above and also enclosed beds with a similar insulated tube with the whole rootstock/scion covered in damp peat moss etc.
I’ve found the biggest issue is keeping the humidity high and constant so the scion doesn’t dry out, this Jan/Feb I’ve been at 60-80% take on walnuts on juglans regia stocks, most of which are 12-18mm (1/2"-3/4") diameter.
The heating cable and thermostat wire are just hung around the inside and I use damp perlite around the rootstocks, I’m testing whether to cover the whole scion with it, making sure it’s not too damp, plastic bag over the top which ensures about 90% (ish) humidity.