Hey i understand your pain,i made several ATBA,maybe 3 with different material and design,eventually i reused all my materials to make this.
http://cameroid.com/i/2WSJR-A1
It took 10 minutes for them to go bask on it after it was done,even tough they would never try my ATBA.
Proximity to water is important,so they feel that they can escape easely.
The heat must come across the ramp,mine wont even think about climbing until they feel that big beam of heat in the face first.
The heat must be directed on some absorbing material,like a rock or ceramic tile.but not too warm.
This is so far the experiance i had with my 2 turtles,Hope it help out.