Hah, this brings back memories. First you can grab some small wine fridges online for about $150 and work pretty well as long as the heat differential isn't >50'.
We tried styrofoam, we tried lacquering actual truffles, none of this worked well. What did end up working well is getting clay from an art store and making the same size as our current line, then dipping, decorating, and then spraying some acrylic on that. They last for about 3 months before you need to peel the skin off and remake them. We used these during the summertime farmers markets when we needed to be able to show product w/o compromising the integrity of the coolers.
The only other option we thought of was to make custom wax molds but the cost for such small orders was really out there. You'd get the look and temperature holding 100% but unless you're getting 1000 made it just doesn't make sense.