I made a version of that and haven't used a tank heater in 2 years (Colorado). Anything you do to limit exposure to outside temps is going to improve it's performance. First put the tank on insulated OSB board, cardboard, straw... anything to get it up off the cold ground. Build a large box that the tank fits into. I found that the panel they use was rather inefficient for solar gain. Just go completely enclosed on the sides for ease. Stuff the dead air space with more straw or foam peanuts. (I fill garbage bags with the peanuts and stuff them in the corners). Most important is cover the top. Even if it's the only thing you do. Make the drink spot as small as you can for your horse to comfortably fit it's head into. I go with a 20" opening. You can keep the tank heater in the tank if you want but it will be more efficient and run less just because you aren't getting the heat exchange with the air. Finally paint everything black. This will help absorb as much heat from the sun as possible.
I found some 2 1/2" foam insulated OSB board from a Resource Store (building seconds) for $8 a sheet. Need 4 for a 100 G tank. Piece the bottom with the off cuts. Used scrap 2x4, bought 1 G of black paint, an old torn stall mat (for the top around the drink hole where they dribble or I get ice build up on the top). Spent less than $75 or about the same as a new quality tank heater.