That might warp the benefit/effort ratio too low!
Clever idea, can't hurt to try it. Don't expect miracles (hopefully, no one is!), with the speaker already mounted in the device, and no real access to the back side, this can't do much, but it could help enough to be worthwhile. Experiment.
One comment said to use a book to raise the phone off the table. I noticed my cheap tablet has a tiny little speaker that sounds much better (still marginal) with the tablet laying down on a hard surface. I think they optimized the layout for that.
I do plan on buying a little 12V amp, and re-purposing some old speakers I salvaged from an old Apple E-Mac (amazingly good little speakers, with their own plastic resonant chamber), and adding a 12V SLA battery to make a lunch box sized 'boom box' (w/o much 'boom), so I can use my tablet portable for playing podcasts. I like being free of the headphones and charger cords for that. Don't need big sound, just want to hear it at normal speaking levels.