How big is the tub? Here are some suggestions:
First, try insulating the tub with a blanket. If that works, you can figure out a more permanent/attractive insulation strategy.

You could get a second UTH for the cool side on a separate thermostat.

You could get a CHE or heat lamp positioned above the lid of the tub, although doing that always seems to me like a whole lot of heat is going to waste and you're putting something that gets pretty hot next to the plastic lid.

If the second UTH doesn't work though, I think the better solution is a radiant heat panel. The panel distributes its heat output over a large area so it never gets anywhere near as hot on its surface as a CHE or an incandescent bulb. I have used one to maintain a perfect ambient temperature in a glass tank in a room that is 55-60° in the winter. The panel will also work well in any other style of enclosure you might switch to down the road too, so even though it's a more expensive investment than lamps and UTH's, it will continue to serve you well in whatever other kind of cage you use later.

Using a space heater in order to heat one little box is probably the most inefficient thing you could do. Add up a few months' worth of electricity for that and compare it to the price of a RHP and the RHP starts to look like a pretty decent deal.