I used depleted uranium as the fuel, the output will drop to below 0 watts when you first change it but it also produces a lot less heat. Just adjust all the cooling down until you can reach the power you need.
Here's the lightest "efficient" thermocouple i can build so far:
Sadly, it still needs about 25m² of maximum theoretical efficiency radiators. Even increasing the exit temperature to 510 K (and using silver as a heat conductor so you actually manage to have a reasonable length), that's still 15 m² worth of radiators.
We need RTG fuel quantities smaller than 1kg to make them viable cost-wise:(
As for a heavier alternative with a saner exit temperature, there's this:
It fails miserably in terms of mass efficiency because both thermopile materials are (relatively to the amount of heat available) good heat conductors and need that much surface area to achieve the intended temperature delta.
Here's a less efficient version that takes advantage of the low thermal conductivity of uranous oxide to work, and manages to get a higher operating temperature:
If you don't care about cost, RTGs still perform better mass-wise at very low power usage. Sorry.