Nobody makes one. I could foresee two different options, only one of which I would undertake.
By a well talented blacksmith, the trigger guard could be stretched and reshaped - however, this would leave the trigger guard thinned, so I wouldn't play with this.
Alternatively, there are plenty of guys who can expertly weld stainless, so it wouldn't be a challenge to cut off the existing trigger guard and have a newly fabricated, appropriately profiled trigger guard be welded back in its place. This would be the route I'd take.
However, all of that said - when temperatures drop, I use flip top insulated mittens, a combination of liners and an insulated glove/mitten shell, or use "riflemans" gloves with a thinned trigger finger - else I'll cut and resew the trigger finger, and wear a simple liner glove underneath. Most of the time I don't wear much for gloves at all anyway, and rely upon a muff to keep my hands warm enough until I shoot. I'll often lay my revolver on my lap with the muff laying on top of it to keep the steel warm as well, or when walking, I'll wear my chest rig inside my insulation layer, so the entire revolver stays warm. I've hunted predominantly with a revolver for all of my life, including several years living and hunting in north country like Michigan, and I've never had any problems keeping my hands warm.
I'll also say, there's no way I'd stick an insulated finger into a Ruger SBH trigger guard with a glove so thick for hunting. It's one thing for a fighting rifle to have an enlarged trigger guard and be used with insulated gloves - with a 5.5-8.5lb trigger and a torso sized target, but for hunting, the bulk and compression of an insulated glove combined with the 2.5-4.5lb trigger pull of a single action SBH would be a non-starter for me. You'll have no idea when that revolver is going to bark.