Remove some materials from the inside of the grips.
You can bend the mag brake (the flat piece you referenced) so that it is bent away from the mag. With factory grips, the mags should drop free; if they didn't before modifying the mag brake, they should afterwards.
Generally, its best not to remove the mag brake, as it keeps mags from catching on the mainspring. Not a big deal if you do, but better if you don't.