heat loss from your mash tun will only be through the outside walls, bottom, and top of your tun.
Insulate better, or alternatively you could wrap a low wattage heater around the outside of your tun (like "Flexwatt Heat Tape" used for reptile tanks) under the insulation.
Personally unless I was going HERMS, I would concentrate on insulation. I have insulated my 30L keggle (pony keg) mash tun with 1.5" thick ceramic wool ducting insulation on the sides and bottom, with an insulated lid on top and I lose about 2 deg. F in 90 minutes.