If ya ever got a question 'bout buildin' your own thunder box, get in touch with me. I'm more than happy to share what I've learned 'bout em. Since doin' this well before the turn of the century, I probably properly built more of these than most anyone else and don't feel the need to compete with anyone. There is a great deal of satisfaction in doin' it yourself, if ya can an' I'm happy to give ya advice ta make sure ya gots a proper one to be proud of. Take advantage of my age and experience 'cause my rustic American kybo expertise should not die with me.
The pit should be at least 3 feet deep, but ya gonna want it deeper.
Make sure it is a good distance from ya water source and the bottom is not on bed rock. If ya got local septic distances use that to figure the location.
Dig it with a backhoe, post hole digger, ya daughter's beau or whatever else ya got but use a 55 gal. drum, trash can or just plain durable hardwood to stabilize the top. Ya can't dig it too wide if once the drum, trash can or whatever is in place ya simply fills in around it.
1) If ya gonna use a shovel measure out on the ground a distance a bit longer than the depth you want the pit to be. Than start diggin' from one end of that mark, at a good angle goin' down all the way until ya reaches the other mark which, if done right, should put ya at about the depth ya wants. Ya should now have a long narrow trench.
2) Take a metal garbage can, fifty-five gallon drum or anything similar and poke a few holes in the side.
3) Stick that can into the deep side of the trench so's the top of it is at ground level. The bottom of the can don't have to sit on bottom of the pit because this is used to stabilize the top.
4) Fill in the un needed part of the trench and ya should only have left a top of the pit, the size of which will be the size of the can top which ya can build or place ya privy over.
note: I don'ts dig pits anymore 'cause I figure that some day when I'm diggin' near the bottom, some fella might walk by, look down and see my old face, figure I'm dead an' fill in the hole.
Watch my YouTube Outhouse Building Tips #1 on the Outhouse Americana channel. This video tells ya good structural and ventin' tips. I tend to ramble a bit, but if ya puts up with me and watches the whole thing it'l be worth it for ya.
The seat can allow the most gasses into the sittin' chamber from below. Whatever ya uses for a seat, construct a simple airtight cover to seal it when not in use.
If ya uses a hardware store seat, remove the spacers from the bottom of the seat, caulk it and attach it to the bench. The cover what came with it ain't no good because it has spacers which will allow gasses to escape through. I discard em in favor of an easily made air tight cover of my own. Ya might wanna give that old cover ta aunt Sophie who can paint folk scenes on it to sell to tourists.