I’ve used a few of the mesh brass catchers out on the market that attach to your AR-15, and for the most part they kinda suck. There’s currently one on the crowd funding site RocketHub that looks like it doesn’t suck. Called the Brass Wrangler, it’s made out of Kydex and snaps onto a Picatinny handguard rail. The bag that holds your spent brass is then held into place by your ARs dust cover. The only drawbacks are your AR has to have a handguard with a rail up top and a dust cover for it to work properly.
To get a Brass Wrangler of your own you’d have to fund at least $35 towards the goal. Note, that isn’t an MSRP, but the minimum amount to contribute to help the creator produce his product. Check it out over at RocketHub.com.