I have a home brew composting toilet on my weekend farm. It has been in use for about 4 years before I emptied it a few weeks ago. I simply dug a good size hole and built a small shed over it. I then got some formwork ply as a floor. The throne is a 20ltr drum with the bottom cut out with a box built around it. This is screwed to the floor which has the appropriate cut out.
It has a chimney coming from under the floor with two 12v computer fans. One is attached to a small PV panel and the other to a controller that turns the fan on for 1 minute every 10 minutes. This is powered up when I turn on the battery bank for my camp.
Next to the throne is a bucket of sawdust. You can work out what this is for!
The picture looks like someone needs to improve their aim but is simply because the hole in the floor is not big enough and moisture dribbles down the inside onto the floor. The new floor is better!
This has handled weekends with over 20 people without any complaints of smell. The material removed was just crumbly compost that I buried nearby in the bush.
You probably will need a permit for a working outhouse and they are about as easy to buy as hen's teeth. I think I would build something that looks like a small portable shed. It'd be less likely that the DNR would be interested that way.
Hole in the ground is lined with an old water heater tank with the ends cut out. Toss a little wood ash in the hole occasionally for odor control. ~~ Lowell
Seems like it depends on who and how often the convenience is needed. If it was just me on the back of the "farm" a bucket with a seat adapter, a couple tires over a hole, a log.......just about anything would do er do do.