put fake burrows (like sunfox has) in. With the amount of sand you have, you could probably do something pretty elaborate. If I were you I'd get three of those ziploc containers. I would put one in the center of the tank against the back wall and then one on each end of the tank. Then I'd have a tube coming out of the sand as an entrance to one of them. I would probably have the boxes on the side pushed with their longer side up against the side of the tank and one of the shorter sides pushed against the back wall. the middle one would have it's longer side up against the back wall... if you get what I'm saying? Okay, so you have them all in position, where was I? Oh, you have the one entrance tube be probably about 12" long and have it come from the shorter end of the burrow box (the end facing the front of the tank) and curving up and towards the center of the tank. Then have it coming out of the sand just enough so that the uro can get in. Do the same for the other side of the tank. then you tubing to connect both of the side boxes to the middle. I would think this would allow for a pretty good temperature and humidity gradient.
Then you just fill the boxes with dirt (not all of the way of course) and put some dirt in the tubes too. Then, what I personally would do is, have the sand (well, I'd probably use a dirt/mix) that you're using for substrate come up around the boxes just high enough so that it reaches the top of the boxes but doesn't cover them. Then I'd put cork bark slabs over them, that way when you have to water it every couple days you just lift the cork bark and the lid and put some water in. If you don't want the cork bark you can always just cover them with sand like sunfox does. It doesn't make it much more complicated at all.
Also, for your basking spot, I would use a retes stack, definately. They work very nice and allow you adjust the basking temps just by adding or taking away levels. Pro Exotics has a lot of pictures of what a good stack looks like. Since you're using sand, when you put the stack in the tank, I would remove the sand (or push it out of the way) from the area the stack is going in so the bottom level will be right on the bottom of the tank and it will have a good steady foundation. Sometimes with larger uros (in my experience with my uro), when they get in between levels they can actually sort of move the levels above them around, which isn't really very good, but you can stop it by attaching the levels to each other. it's not really necessary, but sometimes it makes it easier if you build it sort of like sunfox did so that there's basically steps to get to the top. You can also have corkbark slabs leading to the top if you like.
That's just what I would do. If there's anything you don't understand just tell me and I'll try to explain it better or maybe just draw a picture. I'm really putting a lot of thought into this because I'm building new cages for my beardie and my uro, and i think I'm going to use this or a similar design for Sam, mali. Oh, also, make sure you provide a dry hide on the cool side too. you can do this with some nice cork bark or something. Cork bark...what can't you do with it? 