I was struck by a slight touch of genius just a few moments ago and had to share.
Anyone who has considered a spunding valve has probably looked at the price and flinched at yet another $30-$40 piece of kit that they don't really need but want anyway. Or maybe not, some folks have the cash to indulge when the fancy strikes.
This is for the first group. Pressure cookers are set up to operate at 15psi.
Purchase a replacement vent pipe and regulator from the manufacturer or amazon, or look at your nearest garage sale and scavenge an old pressure cooker for parts. I just pulled apart the valve on my pressure cooker and found the vent pipe was threaded 1/4 NPT, I'm pretty sure it was a sears model but which one I couldn't tell you as I got it at a yard sale for three bucks a year ago.
Combine that with this sanke keg fermenter conversion and you're set to go. Instead of the hose barb, press the vent pipe into one of the holes in the bung. You may have to jimmy rig a way to get a gas line to it to transfer under pressure, but as the vent pipe resembles a hose barb without the ridges, a simple pipe clamp should suffice long enough to transfer.
Ok, you don't get a fancy gauge to tell you exactly what the pressure is, but so long as the vent pipe is vertical, you should be venting CO2 at or near 15psi. Thanks federal safety regulations!