IIRC, the US Army used to have some OD green canvas 2-clip belt pouches that were supposed to be used in garrison environments, like if you were standing gate guard with your Garand and for some reason couldn't wear the full 10-pocket cartridge belt. I think a bunch of them were given to, or made for, the Greek Army when they were given M1's after WWII, and got imported back to the US a couple of years ago - try searching at
www.sportsmansguide.com and
www.e-gunparts.com for them, that's where I remember seeing them advertised.
Of course, this begs the question of why you need a speed-pouch for a Garand in the first place? It's pointless for NRA HighPower Service Rifle competition, you can either keep a couple in the pocket of your shooting coat or lay them out on the ground next to your shooting mat for the stages that require reloads. If you're going to shoot a 3-gun match or attend a practical-rifle class with your M1, more power to ya, but those activities are really oriented more towards rifles with detachable box magazines. If Thunder Ranch is still offering their Old Rifles Class, I believe they require all ammo carriers to be period-correct for the rifle in question, so that's out. Maybe for boonies-hiking or truck-rifle/home-defense applications?
Here's a cheap solution I just thought up - get a nylon/elastic belt slide with cartridge loops from Uncle Mike's, or scavenge the plastic ammo holders from a box of Federal .30-06 hunting ammo, Federal had them molded with belt slots. The loops should allow you to insert the four rounds on one side of your loaded M1 clip deep enough to hold the clip pretty securely, and with no flap to unsnap, your reloads should be pretty quick. If that carry method works well for you, get in touch with
www.buybrownholsters.com and see if he'll custom-bend something for you in Kydex that'll be more high-speed.