Move the half log back so it butts up against the far wall, in the corner. This gives more floor space. Add a couple of plants, real or fake, to allow him to feel a little more secure. Cut a hole out of the screen so the UVB rays from the light are able to penetrate into the enclosure. Pour about a pitcher full of water into the substrate and mix it up with your hand, then pat it down to make it easier to walk on.
I don't agree on the hot side/cool side theory for RF and YF tortoises (or baby tortoises of any species). I have two light fixtures, one at each end. I have the night heat over the hiding place end and the day heat/UVB over the other end. My baby enclosures are 80-85F all over the whole habitat.
It might be too cold/drafty on the floor. Can you set the aquarium up on a table or something, and cover it?
Heat pads don't work well for most tortoises because they like to dig and it can cause problems for them. A CHE (ceramic heat emitter) hooked up to a thermostat is commonly used. Plus it will help with night time temps.
I use black lights, frowned upon by some members here, but I've never encountered a problem with using them. If you don't want to use a black light, use a ceramic heat emitter. I place the fixture with the black light over the hiding place end of the habitat:
black light (usually 60 watts)
ceramic heat emitter (CHE - 100 watts is big enough)
At the other end, I place the Mercury Vapor Bulb (100 watt is plenty hot enough):
In order to maintain an 80-85F degree temperature, I sometimes have to leave the black light on during the day too. But in your small aquarium, the 100 watt MVB should keep the temperature up, providing you cover the tank.
I would moisten the moss and stuff it into the half log. Don't forget to add more items to provide shade. The harsh MVB will have the baby hiding in the half log all the time if you don't.
This is what the enclosure looks like now. I took your suggestions and have a powersun on the left and a night bulb on the right. It gets about 90 degrees. I also used a few water bottles to wet the ground. Anything else I should do?
Every night tuck the little one under the stuff in the hiding place. It should be slightly cooler in there and a small drop in temp at night is ok. I'm not sure about the 90F degrees. I keep mine between 80 and 85F and they do well at that temp. Have you given consideration to covering the whole thing?