One problem I see is the use of a plexiglass top.

Plexiglass will retain the heat and reduce air circulation. This is not a good thing, especially when you are using a lamp to raise temps to 90 . The result will be an overly hot and humid enclosure with poor air circulation. Your tortoise needs a relatively dry enclosure with both cool and hot areas.

I suggest building a screen top around a wooden frame. This allows for the heat and light fixtures, plus adequate air circulation. I've used heavy-gauge screen or chicken wire over 2"x2"s for an enclosure that's about 24"x40".