My suggestion is to have some sort of geyser or heat vent that acts as a small shield and would allow you to place a thermal reactor on.
alternatively you could use something to capture steam or heat energy used similarly to the ore in which you could power shields further away.
Another suggestion building off of this is adding a heat element to the game in which creep has a preferred temperature and flows faster or slower depending on how close the area is to that.
You could then use the steam or heat to heat up an area making creep move slower there.