I've been fermenting under pressure the last few years. I've tried a few different relief valves, currently I have settled on these:
Pressure cooker jigger valve
They're very cheap, and have a low hysteresis which I prefer. The thread is close enough to 1/8" BSP you can get it in with a decent amount of thread tape. There is no pressure adjustment - it is only fortunate that the pressure these start to let go is about perfect for fermentation. Most of these and other small PRVs like it leak a tiny amount over time - I find it quite beneficial to put a small valve in series with the PRV so you can use higher pressures for oxygenation and transfers if required, as well as simply sealing it off when fermentation is complete. Obviously never leave it closed at the start of the ferment!
I have also experimented with replacing the spring in the built-in PRV in a corny keg, which works OK - corny kegs are a bit of a pain to sanitise as effectively as a 50L keg - those you can just bung on the gas burner with a bit of water inside, and boil away. You can even put the pressure cooker valve to its intended use if you're feeling brave.