Hi, heat mat might be a little small for 3x2, it should be about 1/3 of the floor space and since you have 6 square feet of floor you'd need a 2 square ft heat mat. Try 17x12 sideways so it's only 12in wide but fits better into the 2ft of depth. Heat mats are the best way to heat corn snakes.
Measure the temp on the floor, guessing what it will be on the floor from the temp you get up the wall is not much good.
A stat will keep the temp right. It should be 82-86 F on the floor.
Hope that helps.