On my cranwelli tanks I place the heat mat on the outside of the tank mounted on the side and covering about a third of the tank. Horned frogs often bury if they are getting too warm so a mat under the tank wouldnt help that. I have found you get a great heat gradient with a side mounted mat - plus you can get self adhesive heat mats now whiuch are great.
If you are worried about him burning himself maybe put some cocofibre matting on the sides - makes the frogs feel more secure as well and you can grow plants up them to liven up your tank too (plus plants will help maintain humidity levels)

Agree the heat lamp going is a good idea plus be careful with your lighting with your albino and dont use the high UV bulbs