I'll add another vote for velcro - it works quite well. We used the material from a set of pop-open auto sun shades cut to fit our windows and sewed some black backing material on them. A few small pieces of sticky velcro around the metal window frame holds them in place nicely.
TT: 2015 Jay Flight SLX 195RB Baja Edition
TV: 2014 RAM 1500 Bighorn CC, 5.7L Hemi, 3.21, Factory Air Suspension/Tow Pkg, Andersen WD/SC