...and we're back.
For reference, here's our OEM Subie battery in its cute little jacket...
And here is the label in case yours is obscured by the hold-down or grime...
The OEM battery is approx 9" long x 6 7/8" wide x 8 7/8 inches tall to the top of the posts. The case is ~8" tall, meaning the posts are about 7/8" tall. That's pretty much right on the Group 25 standard referenced above as 9 1/16" L x 6 7/8" W x 8 7/8" H.
The battery sits in a flimsy plastic tray which just lifts out...
... which measures ~10 3/4" long x 6 7/8" wide. It has two locating nipples on the bottom and 1 drain hole but certainly provides no support. I'd be comfortable cutting the sides or ends or both or removing the tray altogether.
I think we could squeeze an extra 1/2" in length (MAYBE 3/4") beyond the length of the tray, between the windshield washer fluid neck in front and some plastic trim in back, so let's call that 11 1/4" max length.
Width I'm not so sure of without something to test fit, as I think width and length may be interdependent when it comes to clearance at the corner of a 3D box. Another 1/8" should be fine so let's go with an even 7" wide max.
Height is where we are limited by the metal structure at the front. From the bottom of the plastic tray to the structure you've got 8 1/4". Remove the tray, maybe bend some metal a tiny bit could probably get 8 1/2". Remeber, this is case height. If automotive studs are a standard length (7/8" as our OEM ones are) that would be 9 1/8". With some shoehorning, maybe 9 3/8" but we'll go 9 1/8" for now.
That give us 11 1/4" L x 7" W x 9 1/8" H.
So I think all of the groups I mentioned initially would physically fit. I thought my trolling motor battery was a G24 and I was going to test fit it but it's a G27 so I didn't even try.
One other crucial point, the + battery cable doesn't have much slack, so I think we need to OEM post placement unless you're going to mod the cables. That means the posts need to be on the inboard side (close to the engine) when the + post is positioned to the rear and - post to the front. My deep cycle G27 has that configuration so I'm assuming standard G24's would as well.
Please check my numbers above in relation to group sizes but I think we have some options as far as what will fit. I don't think we'll know for sure until someone tries it, though.
One other thing I noticed is that although there are standards for "Group 24" that are ~9" tall, most of the specs I see listed (Interstate for instance), list them as 9 1/2" tall which probably means the case is too tall to fit.