Help! Wild injured garter snake

[Myssi24]

Since it looks like Snakey won't be able to be released for a while, I would like to improve her tank as much as I am able. But as my hubby is currently not working, I can't spend much money.

I'm looking around the house and may check out dollar store for a bigger water dish, like Steve suggested. I'm assuming long and skinny works? How long should I be trying for, the width of the tank or more the length of the tank?

My biggest concern is heat. I currently have a heating pad (like one you get from Walgreens to use on your back) which is doing a decent job, but auto shuts off every 2 hours, which is problematic over night and when I'm at work. I looked into them and can probably afford the $10-15 for an under tank heater, but a reastat (sp?) or thermostat is probably too much. I have an old timer I used to use for my fish tank that can be set in 15 min increments. If I'm remembering the packaging correctly, it was for reptiles too. Do you think I could use that to control a UTH if I have a day where I can watch it pretty closely to get it set up?

When I next clean her tank I would like to change the substrate a bit. Because of my heating options, I don't want to just use paper towels. She currently is on colored fish tank gravel covered by moss on the cool side and thin agate slabs on the hot side. I'm thinking of replacing the small gravel with a layer of 1" ish size pebbles a couple inches thick with paper towels on the very bottom to absorb liquid and the agate and moss or maybe pine bark pieces picked up from the wood pile she lived in on top again. My thinking here is the rock will heat up slowly enough to prevent her from getting burned by the UTH, yet cool slowly enough to help maintain the temp since I will only be able to set it on or off. What do you think?

last I'm not crazy about the lid of the tank. Right now it is a glass fish tank lid with the opening in the back for the filter and stuff taped shut with duct tape, no sticky side exposed. Right now it does help keep the heat in. My concern is this doesn't have much air flow. It isn't sealed by any means and I open it several times a day to let fresh air in, but the humidity from her water evaporating is starting to build up. Once I have a more stable heat supply, do you think cardboard with small, pencil tip sized, holes punch in it with small rocks to weight it down would be secure enough?

An other free or low cost ideas would be much appreciated! Thank you all so much for all your help!