That sounds about right. It should be 90 at the floor where his heat pad is. I would try to get the ambient temps up to at least 80. I have that same size container. And that is why i added the light. The pad wasnt doing the job by itself. Also if you have your AC on it will make the air in the enclosure cooler. I think i have the same type of heating pad.

Do a test before you put it on. Put the thermometer on the pad and set it to low and see how high the temps get then on Med then Lo. Also If you are concerned about the surface being too hot i would add some substrate the news paper you are using is too thin and doesn't spread out the heat so its concentrated on top of the pad. I use aspen bedding to achieve this.

In my opinion this is what your set up should be.