Photomike,
On all that I've seen there are 2 bolts that hold the toilet to the flange. Just remove the water line and the 2 nuts from the bolts and set it out. Between them there is a foam gasket. The head of each bolt slides into a slot in the flange. Easy to remove. The flange is about 1/2" thick off the floor. The replacement gaskets are cheap.
You can seal it off with a piece of plastic, some type of thin gasket, and using a few 1/2" flat head screws.
You'll have to cut something, 1/2" plywood, to put around the flange to level an area the size of your porta potty, unless it has a concave bottom.
I'd drill a pilot hole for the screws to prevent the flange from cracking, depending on the flange material.
There should be no problem putting the toilet back in. It shouldn't take 10 minutes to change.
You'll also have to plug the water line.
Should work well for your needs.