So, we picked a spot on the slope beneath the house and built a platform beside our solar setup as the base for our shed/outhouse-to-be.
(The funny thing was that the longer I thought about the workbench, the more I thought about using it as a spot to write...)
What was most surprising was that we'd gotten so far on the project without realizing the right spot for a shed was right under our feet. If we built the shed under the deck, we'd only see it when we wanted something in it. And maybe we'd just keep putting plywood over the solar unit on the few occasions we get a big storm.
Could we build it in the woods? We tromped around a little and sadly came to the realization that the spot I'd eyed across the creek was too rocky and we didn't have the right trees to make it a treehouse. Right when I was about to give up on the idea, we realized we wanted to build it on a trailer. Long ago, we'd talked about having something smaller and easier to tow that we could potentially relocate, if we ever wanted a temporary change of scenery.
So that's the plan for now: find a used trailer (if possible), draw up a revised sketch and build our studio/guesthouse/outhouse/glamper. And we'll try to remember our lesson learned: Unless you build on wheels, the site has to be just right.