I'll have to look at my toolbox again, but I got a regular socket that works--IIRC it's 27 mm. But this should provide the info you need:

 

Thanks, Bill. 27mm it says, so that’s what I’ll get. Gonna check if there’s a specific filter socket, though—I think there are some that sorta clip on.

I kept meaning to measure it, but it’s hard to get my caliper in there when it’s installed, and then...there I was, wandering around the shop with the wet filter cap in my hand looking for the damn caliper (of course, I’d forgotten to find it before removing the cap), and as it was starting to rain outside where the Jeep was, I figured I’d better get it installed asap (don’t tell me this kind of thing never happens to you!).

 

You want a 27mm shallow socket. They are usually labelled as oil filter sockets, and they do sort of lock onto the cap.
Here's one example.
I got mine from O'Reilley's.

 

Thanks! Yeah, this is what I was thinking of—just didn’t know the size.

 

While you're shopping, you will want at least one socket extension of 8-12 inches and a U-Joint adapter. I've had the best luck using the socket, then a 3 or 4 inch extension, then the U-joint, then a 10 or 12 inch extension.

This allows you to get the necessary angle onto the filter cap, and have the actual wrench up out of the engine bay so you have room to move it.