Fabric will work. We did a lot of psychedelic fabric back in the 70's. At low volume playing it muffles it a bit but not much. Real grill cloth let's it all through...
You need to be able to "see" thru it somewhat. It's suggested that you want something that's about a 50% open weave. One example of what not to do is the tweed Gretsch G5222 amp (not to be mistaken with the G5222 guitar). Tho it has an historical precedent to an early '50s amp, the tight weave with the added rubberized décor on it seriously chokes the sound. Any nice open weave should do.
I was alive during the 60's. It can be done. All of the advice above is good! On a personal note, unless this amp is wedded to a studio or bedroom, I would go with actual purpose made grill cloth. There's the sound part, but there's also the abuse part. Grill cloth usually has a combination of light and heavy threads. When your amp is sliding in the back of your truck, or, more likely, that drunken bar patron may slide right into your speaker. Throw up on it. And then jam his glasses through the cone. Regular fabric may not be resistant enough.
On my late '50s Gretsch 6163, the grille was actually a flocked screen wire thing. Pretty tough material, but impossible to clean.
If there’s any local amp builders see if they’ll sell you a small piece. A guy up here from New Vintage Amplifiers charged me about $15 for enough to do 3-4 1x12 cabs.
I used as one outdoor furniture cloth from a local fabric store on an amp. Sound didn’t seem affected at all.
Speakers are moving air, so As long as you can breath through the weave, most any material could work...? Speaker cloth is durable. Pulling the baffle can be a biach... lesson you got Velcro type. I’d make a custom baffle to recover n keep the stock OG.
I just bought a big roll off a friend, proper Fender wheat cloth.... I’m not sure my attempt at attaching it to the baffle was my best work. It needs to be tight I’m gonna try the hairdryer shrink to fit technique next!! Luckily Mrs Moo has one as i really have no need
So how much do you need? I've got some oxblood I used redoing a little Fender Champ, same as that G5222(I think).
That is the same amp inside. Better yet, the Fender goes to 12! Oxblood would look great with that brown/tan cabinet. Oxblood would look good on the Gretsch G5222 amp (not to be confused with the Gretsch G5222 guitar) as well. For tight grille cloth, staple at the top center, then stretch and staple at the bottom center. Do the same to the sides. Work your way to the corners. That keeps the cloth taut, and keeps the pattern from wavering. First thing to do is to paint the mounting board face black so the speaker doesn't stand out.
Sure ! That's what I did on my G5222, plus satin clear-coating the tweed : I should never had let it go... A+!
I'm going to be generous and assume you're not a "loser" and TONE is important to you and that GAME-CHANGING "gear" is something that MATTERS to you (quotes, caps and a hyphen, thank you very much). If I'm right, then the only thing you need is E.C. Collins TRUE "BLUESBREAKER-PINSTRIPE" grill cloth. Clapton, Page, Hendrix, etc. You thought tone was in the fingers? You thought it was talent and dedication? Well, then you're an idiot. TONE comes from grill cloth. https://bluesbreaker-pinstripe.com