Summary

I believe it will also work fine on an Ipad 2 but not tested.
Held on with 4 small magnets, self adjusting, takes a reasonable knock or simply slides off. Makes a HUGE difference in audio.
Prints flat in 2 parts with no support needed, there are no overhangs.
I have tried a few out there and they were all a pain to print and never attached properly.

Instructions

Magnets used were 4mm dia x 1.5mm thick.
Stick them to ipad first and mark the top so you know which way to glue them in.
The magnets in the top section can go in full depth but the ones in the bottom section need to protrude a small amount at an angle for best grip.
I used hot melt glue to assemble the 2 halves and glue in the bottom magnets, the top ones I used a few drops of acetone in the hole and it appears to hold them in. Time will tell, superglue is probably better.
When printing make sure the holes are upwards on the top piece.
Model files included for Designspark Mechanical.
Gave inspiration to the Ipad Speaker Chip as I did make one but found it too flimsy, I had tried magnets a year ago though, obvious choice since an ipad 3 with a smart cover will erase a hard drive a mile away.. 37 magnets I believe !
Also a nod in the direction of the Ipad 2 Sound deflector, which I have used for well over a year with rubber bands to try and help it hold on..

Ipad 3 Magnetic Speaker redirector