Newbie here, but plenty of audio background (former Dolby engineer, sound company owner, etc).
Someone hacked up the package shelf on my '66 coupe to shove some 6x9s in but they didn't really fit. The hole is closer to a 5x7.
What I did was make a piece of plywood that fit under the package shelf inside the trunk. Screwing speakers to wood is always better than to sheet metal. Lined up the holes with the openings to either side of the factory 6x9 opening. Bolted the plywood through the rear deck sheet metal. I cleaned up the holes in the cardboard package shelf and covered the entire piece with black cloth. Found some nubby textured flat black cloth at the yardage store. If you want to know if it's suitable for grillecloth, blow though it. If you can feel the air on your hand going though the cloth, it will work fine.
Other than the shelf being covered with somewhat stretchy cloth instead of carpet, it's totally stealth. I put some proper speaker grillecloth over the opening at the plywood level just to protect the speaker from anything puncturing the cosmetic cloth on top, and add a bit more UV protection for the cones. It's not a daily but the sun beats up read deck speakers.
If I ever want to get silly, I can put an 8" sub under the 6x9 opening. For right now, the Polk 5x7s are adequate for listening to music in a noisy car. Together with the Classic Stereo twin 3" speakers in the dash, I still have to figure a way to stealth a couple tweeters at the A pillars to add some stereo image width.