Heating large enclosure

[Brently]

Okay so I have a homemade wooden enclosure 8x2x3 roughly. I have a 160 watt RBI heat panel. The heat panel seems to be having a harder time heating the enclosure than it used to. I am trying to find a economical method of supplementing heat to help. I am wondering if heat rope under the substrate would be adequate. I am currently using exo terra plantation soil. My thermostat is a herpstat 2. The enclosure is already built and I would prefer to not have to do any build mods to make this work. Here are some pictures to give an idea of what I am working with.