Well your idea probably won't work as well as you think. Lets say you wanted a low-ish watt TEC, So I'd say maybe a 12706 so that would be a Qmax of about 50w at 12v, Only problem is that's also about 45-50w of heat dumped with no load. Okay so air cooling doesn't tend to be so effective when cooling high loads so you wanted to dissipate 50w that's about the max that TEC could handle. So you'd end up with a dt of 0 and 100w to dissipate with that heatsink. Lets say room temp was 20C and the heatsink/fan was 0.3C/w so that's 30C over room temp or a cold side temp of 50C. You arn't going to be doing any cooling with that. See the problem now?