Interesting responses! Several different solutions. In my case, I think BruceMs suggestion of the masonry muffler maybe best. I would love to try the leach field set-up, but that is impossible in this location.
One of my friends has offered a very large muffler off an excavator. I tried it in a lash-up but was not satisfied with reduction in the exhaust note. I think an automotive silencer will clog too easily after seeing all the soot that I collected with the Pit.
Recently, we tore down an old building with the intention of building a woodworking shop space along with a room for a couple of freezers and bee equipment storage. Working in between this incessant rain, we have managed to get the walls framed up, and are waiting for the wind (blowing a gale just now) and rain to slack off so we can get the roof joists up. Goes a bit more slowly when you are operating at geezer velocity!
This is where the Dursley comes in to power the building. I will give it its own insulated room, unlike the Listeroid, which lives outside under a shed roof.
In tearing down the old place, I was able to salvage some chimney blocks..... five of them, concrete, about 17" square on the outside with thick walls. The blocks are 8" high. I figure on sandwiching some 2" thick concrete slabs with offset holes in them as baffles. Even have the old cast iron clean out door. We really like working with what is on hand, and most times have to do that anyway.
I have all the materials laying around, so might as well put them to use. Seems like it could all go together without mortar, so could be easily disassembled for cleaning/inspection.
This all will take some time, I will let you know how it works out.
Cheers,
Hugh