A Milt Sparks Criterion makes the Glock 21 concealable. I replaced the rear strap with a Blade-Tech "Pull-the-Dot" rubber strap for additional forward cant.
Regarding the trade: don't do it because you will regret it. Save up and buy a G30S, replacement sights, holster and two mag holders all at once. You may want to get one IWB mag holder because the mags are so wide (Sparks IPS works great). The G30S is a baseplate shorter than the G23 and is a little bit easier to conceal. The Milt Sparks Criterion makes the gun disappear.
Keep in mind my G30 g4 does NOT fit in a G21 g4 Criterion. The G30 slide geometry is different. The G30S may fit; call Milt Sparks to find out.