Help with our first tank

[stephford6610]

We just adopted Medusa, a Yellow belly BP. We are so excited to bring her home, and we are trying to get her enclosure ready and we are having trouble with heating the tank. She's only a year and a couple months old. We have a 30 gallon glass enclosure, 36x16x12, with a metal "screen" top (I put that in quotations because it's not like window screen, it's thicker than that. More like a metal sheet with holes in it).
We purchased a 100 Watt ZooMed bulb, and have it in a zoo med dome, and it's sitting on top of our lid, pointing down into the enclosure. We covered the lid in aluminum tape (outside only) to keep the heat in, with just a small area open for venting, and under the bulb is open.

We have 2 probe thermometers/hygrometers, digital in the enclosure. The highest we have been able to get the temp reading to be is 75. We used one of those gun looking thermometers and the surface temp of her warm hide was 85, but the ambient is just 75. The cool side ambient is only 72.

We turned the lamp off before bed last night, and this morning the tank was 68.

What are we doing wrong?! The guy we adopted her from, who raised her from hatchling, said he has a 70 gallon tank and is able to adequately heat it with just that bulb and dome.

Our house stays between 65 and 70 degrees during these cooler months.

Thanks for the help!