Lying in bed last night, my mind going over the cellphone thing, I thought I might try to make a collapsible cardboard thing like a box with no top or bottom. Then, with two rubber bands, it could be held onto the cell and when finished, folded and put away in the pocket. It wouold have to be just a little bit shorter than the image, because otherwise you couldn't touch the release. Alternatively, the focussing hood from an old Rollei or Mamiya might be suitably inexpensive and worth messing with in this context.
However, theoretical games aside, I've just tried the Samsung with my Schneider transparency lupe X4 and though it works, it's still a waste of time. Moving anything around on that screen brings up different and unexpected menu things. Frankly, if you have to touch the screen to expose, anything is a compromise after that.
Rob C