I'm used to really big stereographs where you have to cross your eyes rather than look to infinity (or even let your eyes go outside the normal limit, like the Simpson's).
Can you add a switch to reverse the images (right image on the left and left image on the right)?
When I zoom to 200% (images 100mm on a side), I can't get my eyes to bug-out enough to get it, but I can trivially get a reverse image by crossing my eyes. I suspect most people could view almost full-screen images this way.
I know it sounds weird to recommend "crossing your eyes", but it is basically what you do to view near objects anyway.
See the discussion of "wall-eyed and cross-eyed" under the "autostereograms" heading of the wiki page
. Also, the image to the right there is for cross-eyed viewing—read the caption (and try it of course). Cross-eyed allows bigger images to be viewed.