A quick internet search revealed the doggy dooley, and a little more internetting revealed a couple of DIY versions that were the inspiration for the job.
My first thought was a large (like 8") PVC pipe and cap. But the local big box hardware store didn't have anything large enough. I wandered into the landscape section and found these NDS irrigation control boxes. Perfect! No modification required, and this one only cost me $11.
The teens were up and eating breakfast by this point, which they took to their rooms so the noise from the jig saw didn't bother them. That's good because they only had 8 hours of mid-summer daylight left and I didn't want to ruin any of it with my project.
The lid is great. It twists into a semi-locked position and can easily be opened and closed. The yard is presently free of all dog poop, and the smell and flies are quickly dissipating.
I also figured that, worst case scenario, if it doesn't work as a septic tank and just fills up with 55 gallons of poop, I can pull the irrigation control pod and cover the whole thing under a foot of soil.
Best of all, I have three teens who never have to suffer the degrading injustice of picking up dog poop with a little bag again. #lifegoals
NDS irrigation control box $11
Septic starter $12
For the gravel bed, I used material I had from another project.
Drill with 1/4" bit
Jigsaw with metal blade