Regulating temps

Hi everyone, I'm a new BP "mom". I know that my set-up is not ideal, but would like some suggestions on how to make it better.

I have Kashmire (a baby BP) in a 20 gallon tall tank with a screen top with clamps. I have a towel over the cool side to help keep heat from escaping and a basking lamp (50 watt) on the warm side with an under tank heater on that side as well (Exo Terra Heat Wave) for a 20 gallon tank. He has a hide on both sides and a ceramic water dish in the middle.

I plan to get a digital thermometer this weekend, but right now I'm using those Exo Terra Dial therms and Hygrometer. I have the therms on each side about 1/2 way up the tank (which I now know should probably be lower to get more accurate temp reading closer to the bottom of the tank). With them half way up the tank, I know my temps are reading too low and not sure what I can do to improve his temps. The humidity is staying around 65 to 70 and the temp on cool side right around 75 and warm side around 80 to 85.

I use coconut bark substrate.

So, having to work with what I have, what are MUSTS that I need to get to improve temps for Kashmire? I read someone actually took aluminum foil shiny side down to cover 3/4 of their screen top and taped it on and helped them. Or get some sort of lucite (I think that's what I read) from Home Depot and get someone who has a jigsaw to drill a few holes for circulation and then cut out a hole for the basking light to help keep in heat. They set this over the screen top.

What brand of themometer should I get? Where can I get it? Isn't there a digital combo therm/hygrometer? Is 50 watt enough for the basking light for increasing temps?

Eventually I want to purchase from Animal Plastics, but for the time being have to work with what I have. Any and all suggestions are gratefully accepted!

Robin