TO the OP: Yes, restricting the overall airflow will help keep the heat IN the tank, but this is safer to test without your snake in residence- before they actually move in. So only make these changes while you're right there, at home all day, to oversee the temperatures in the tank, as they won't increase all at once but gradually, & you don't want to come home to find your snake expired from excess heat or with permanent neurological damage. OK?
And btw, where is your probe that connects to the t-stat? Another reason this is best tested out before you have the snake is that constant checking is disruptive & a bit stressful for your snake- but you're sorta stuck with that now, so do the best you can.