I found some 2' x 2' squares of drywall / plasterboard at Home Depot for $4-5 each. I think that's an even better insulator than wood. Putting the heating mat on top of that directs all the heat upwards.
(I covered my heating mat with plastic film even though it is waterproof - "just in case".)
What's harder is to keep the heat from escaping upwards in a draft and by evaporating water from the seedling tray. A humidity dome helps, but if you have excessive water, it will evaporate, condense, drip down, re-evaporate and so on, carrying heat away with every cycle.
But you don't need to trap ALL the heat - and if you did, you might make the soil too warm for seeds like lettuce that need it cool.
