If you go the welding blanket route avoid cheap ones made out of fiberglass, They shed particles and that's not something you want around food or equipment( just handling one makes you itch).
Look for silicone ones.
How snug is the fit on the thermal blanket? I have one of those Quick Coal doors that sticks out 4-5 inches from the cooker when you include the handle.
Don't forget Harbor fright! Bought a cheap welding blanket and bungee cords for a winter turkey cook last year. Didn't spend more than 60$ and temps held overnight!
35 degrees with a 12-15 mph breeze. Bungeed myTraeger blanket around my smoker. Below the lid flange and above the door. Gained 35 degrees and I can still use the lid and have access to the door and the vents.