I realise it's just a test rig right now; a few things to consider in later versions:
Isolation of the heatsink from the cold finger - try to use nylon screws and nuts as they will not transfer heat the way those steel ones will. Most RC Hobby online sellers have nylon fasteners available. This will improve efficiency. The only direct contact should be through the TEC plate, everything else should be isolated.
Isolate the fan from the heatsink by the use of silicon pads (which most good CPU heatsink/fan combos have these days) this will prevent vibration transfer to the sensor. Even bedding the fan onto silicon blobs on the heatsink works pretty well as isolators. Those screws shown will transfer vibration to the coldfinger.
Where have you put your temperature sensor (or did I ask this before)?
Will be interesting to see how your test data looks. If it's a 12V TEC I'd be running it at that, or pushing it through a PWM where it will still get 12V but current gets varied, I don't know if you will get much out at a lower voltage. Rowland would know.
