What are you using for bottoms?
There was a product called Sole Saver. Used to be at Black Elk in Anch. An old guy on the east coast made it. Fantastic stuff. Went on like cake frosting. But he died and you can't get it anymore that I know of.
I have a gallon that I saved. Some day I will make my own custom otter mukluks. I have everything I need except the time. All my own tanned otter etc.
My first pair I put otter fur on the bottoms. I then put some rivets in them with the numbs out for traction. They worked okay but slippery still on snow.
The second one was better. I made the sole out of otter and turned fur inside. Then coated the leather with sole saver. I made a 1 inch strip vertical around the outside perimeter of mukluk. Coated this too. My mistake was I ended the sole saver at the joint. And that is where they failed after a few years of hard use. I would now shave the leather up past the joint now and coat that too. They take a lot of abuse. I used chrome tanned leather and it held up well. But the seam is too important not to keep guarded.
I used a full felt insole as the pattern out of material. Then inserted the full felt insole inside to give additional insulation and some stiffness. They don't have to be exact as kids are growing. Make them bigger.
I just left an opening in the back big enough to accommodate getting the foot in and then closed up with ties. I think this one thing was responsible for taking my kids out in the cold more than anything. Just hard little insulate feet with new plastic boots. I put a flat felt insole on the bottom. Then a full felt insole in side.
My son and I are talking about making two pair as a project together when life slow down a little bit. I have some nearly knee high felt insoles waiting.
Ayakulik turned me on to another cool option for soles he found that I want to try. Using 3M 5200 chalking for the bottom. Anyone that has been around the marine boats knows this stuff.
Really pursuing mukluks is something that is really hampered due to "What to do with for soles?" problem.
This could be the new answer. Who is going to be the first to try it?
https://lostcreekadventures.wordpress.com/2012/11/23/repairing-steger-and-other-mukluk-soles/