Wool socks work well. Old or new in black. I use "Smart Wool" Crew length. Cut to the length you need if necessary. Using double, i.e., one sock stuffed inside another, is better. Easy to sew up the cut ends by gf/mom with a sewing machine or by hand so they don't unravel.
You can also use golf club covers in smaller sizes, like the ones for putters. IMO, wool socks offer the best solution, even better than a stack of towels over the knob. Wool is a better insulator, but with the interior of your car pushing 120 degrees, nothing will be perfect.
These are El Cheapo solutions and work pretty well.
I have one of the Muz leather covers with a drawstring. It looks nice and you can shift easily with it in place. Only problem is that it doesn't do much for keeping the knob cool -- still too hot to touch at times if the car's been in the sun for more than an hour or so. This is in Georgia, and like Florida we gots HOT rays! BTW, my shift knob is solid Titanium and Aluminum is even worse for the hots!
