The grilles from cars the JBL system have a larger opening and are OEM. They work well for 3.5" speakers.

 

Perfect. thank you sir.

 

I have installed the 3.5" Infinity speakers. I did use the JBL covers, and you'll find that even that opening is not near a 3.5" opening. You'll find the JBL openings only about 2.5". To compensate, I added "cones" to the grilles. I cut/made the cones using these from Amazon:

Cone material

One of the cones that I added (in the pics) is smaller, as the available depth was limiting.

The other option - that I first installed, were the Infinity tweeters. Quite an impressive change. But I thought I'd try the 3.5" units, and they're more to my liking.

I also added in-line resistors (just 1 ohm, I think) to better match the 3.5" and the 6.5" door speaker outputs. Also added bass blockers, as I installed an amp, and didn't want the lower bass through the 3.5" drivers.

Are you doing an amp?

 

I will replace the front speakers. In the first stage I will install 2.7-inch Unity U2 drivers to replace the stock tweeters. Did you find a speaker connector (preferably form Aliexpress/Ebay) to help you with the wiring?