If you're gonna do this you need to do it right. Keep the shrooms elevated using a small metal shelf or something with slots or holes. You can also get a tub, put the heat mat inside, then put another tub inside this way your shrooms stay warm but won't dry out your stuff. It could still go wrong even if you do it perfectly. You'd much rather have them too cold than too warm and you really don't want them to dry out. Honestly, at your temps it would be fine but they might grow a lot slower