I've had some modest success improving the cabin warmth of my RV-9A, and I owe it all to the various posts here. So I thought I would list what I've done, in the hope that some other freezing RVer might find this info useful in the future.
1. Sealed up the eyeball vents.
I was getting a ton of wind through my plastic eyeball vents, even when they were "closed." Two vinyl caps, suggested by someone here on the forums, were a quick solution and a huge win in the "bang for the buck" category:
I'm sure someday I'll bite the bullet and get decent aluminum eyeball vents, but for now, these worked wonders.
2. Wrapped the exhaust muff.
I got some exhaust wrap from the aviation department of Autozone and wrapped it around my heat muff. My hangar neighbor A&P signed off on this approach, while cautioning that it's important not to wrap the actual exhaust pipes. I also stuffed a copper scouring pad into the muff to give the air some more hot metal to flow around before entering the cabin (another suggestion I picked up here). The improvement from these changes was noticeable.
3. Heated clothing.
There's a lot to be said for solving this problem with clothing, since it's bound to be cold getting in and out of the airplane as well.
I got a battery-heated vest for my intrepid copilot, and she raved about it so much that I got one as well. This is one of those rare "yeah right" gadgets that actually turns out to be super useful in real life:
Prices appear to have gone up since I got ours, so you may want to hunt for an equivalent by another brand. I'm guessing there's all sorts of heated clothing pitched at motorcycle folks.
4. Hideous hat.
The reason Elmer Fudd wears an Elmer Fudd hat is because those things are..... AWESOME.
I use in-ear Halos for my headset, so I can fold down the ear flaps on a Fudd hat and be super comfy while flying. The long bill on this one is a big plus under an RV canopy, especially because you'll likely be stowing the overhead shade in order to soak up sun.
I got a 2XL and amazingly it was just on the verge of being too large (my head is so huge, I don't have a forehead, I have an eighthead....)
Key thing to keep in mind: nobody can see your fashion up there. Just remember to take the Fudd hat off before landing or crashing, so that you aren't caught alive or dead wearing it.
My next projects:
1. Tackling the gale blowing up from the stick well.
2. Sealing the rear of the canopy. I'm still getting a bracing little breeze hitting the back of my head. Starting to think about how I can do this cheaply and easily, and in a way that might be removable during the summer, when having the breeze might be nice.