The dome looks too narrow to me...What size is it? 10" domes are the best for spreading heat out more evenly, so if you have a smaller one, you may want to exchange it and see if the larger one helps even out the temperature. As you said, it might also help to put some blankets or covering around the sides of the cage to help hold the heat in more.
If you're still having trouble even with that, it may end up working better to get a second lamp & CHE, one on each side of the enclosure, to help heat it more evenly. You may only need 100 watt CHEs if you do that too. On the plus side, glad you're checking all of this out before you get your hedgie! Gives you a bit of time to get things settled before he/she is in there.