you can use a grow light (about 50 w for 30 gal) you'll get about 5 degrees out of it. I live in wisconsin and also have back of viv and side wraped in felt

I'll just tell you what I've got going on for a 33 gal
It's got a under tank heater with very little substrate over it. I used rock because it seems to keep the heat
I have a human heat pad on sleeping side of the tank, its on low with a few hours on medium for a few hours in the morning. they sleep right on the mat. So I have a therm strip on the glass so I know it's at 86 degrees--
And a grow light at 50 watts in the warm sleeping corner
Plus a hood grow light at 2.0
And it still takes till noon for the bient temp to get up to 80-82

Now. Have mine between windows, bad location, but my house is freezing durting the summer, and I have heat in the floor durring winter. Hence, why all the felt, it keeps cold off the wall from peniteating the viv. Id like to think that I can unwrap and take the heat pad off soon, but I doubt it. I have no air conditioning and it stays pretty cool (70) in here all summer.

If you buy a under tank heater, stick it to the side and make sure it's the right size. Couple that with grow lights and you might get up to 80. Don't use reptile basking heat bulbs with these guys, it dries more as it heats

Good luck experimenting, add one thing at a time, start with adome and grow light( buy at hardware store- 5 10 dollars cheeper) the domes are 16-20, and don't set it right on glass