Friends of mine use water bed heaters that they buy at garage sales
for a dollar or two. I think that these are flat and are made to go under the
water bed mattress and keep the large volume of water warm, and seem to do a
good job for them when they start seedlings for their market garden.
I bought one of the sort of expensive one with a seperate thermostat
from the Leonard catalog 6 to 10 years ago, and it is working fine yet.