On my original setup I used a wood table with cork mats under the boil kettle and it worked great. Nothing used under the MLT or HLT:
On my new setup I switched to a stainless table and don't use anything at all. More than a couple of inches away from the boil kettle the table is barely warm:
I did try a couple of large thin silicone pizza mats under the boil kettle but it did very little to keep heat out of the stainless table.