One dimension is the same on all pieces (what turns out to be the depth when installed), but unless you are going to miter the edges the length of the pieces will be different from the horizontal pieces and the vertical ones. I'd recommend cutting a couple of pieces and test fitting them before cutting all of them.
You won't be able to fit the pieces on the front side of the driver quite all the way back against the baffle without causing some interference with the drivers.
A small table saw really made cutting this stuff very easy. The edges were really nice and clean. I imaging you could do it with a jig saw, but it would be harder to get smooth, straight edges and would take considerably longer. If you can borrow a small table saw or rent one from a local equipment rental store, it will be a LOT easier.
I did not take my advice to cut a couple of pieces and test fit, and I ended up needing to trim a lot of pieces slightly smaller to get them to fit. Even so, with the table saw, it only took a couple hours to cut all the pieces (many twice) for two triple subs and clean up the mess with a shop vac afterward.
Cutting this stuff creates a lot of little pieces of foam, so wear goggles and dust mask, and expect quite a mess to clean up.