There's this ingenious little hack video: "How to make tritium nuclear battery at home".
The idea is very simple: "tritium keychain lights" - tubes with tritium and luminophor - placed between solar batteries, they provide light, converted by the batteries to electricity.
The battery in the video provides very little electricity, but it looks like the solution could be easily scaled up - it only uses two tiny keychain lights, $33/pc, and cheapest, dollar-store "garden solar light" solar batteries.
Half-life of Tritium is 12.3 years, so it would not perform nearly as well as 238-Pu with half-life of about 87,7 years on long-term missions like Voyager, but the simplicity, low mass and accessibility are so far superior, it seems like a tritium battery based on thin film solars, of the same mass as a typical Pu-238 based RTG, on timescale of ~25 years - missions like MSL Curiosity rover - could outperform the Plutonium counterpart by strides, and in fraction of the price.
Why isn't this kind of power source not more popular?