OK cool, my back panel is not recessed, only the front, but other wise we're the same.
I couldn't remember what size ply you used, I was waffling between the 3/4 and 5/8. I will have to have a couple extra braces it looks like. Also I don't have any tools that can cut out the circles for the speakers, what did you use.
i used a router bolted/screwed to an offcut of mdf, and then a nail that goes through the mdf and the baffle where the centre of the speaker should be. the idea is to use the router like a compass.
make sure the nail is half the diameter of the speaker mounting hole is the distance from the centre of the nail to the furthest side of the router bit, not the middle the speaker mounting hole is in the spec sheet of the speaker, and is smaller than the speaker size, remember there is the lip that the speaker sits on.
then you turn the router on and adjust it to cut into the baffle a few mm. deepen it only slightly until you get through, and then only go partway around (or better yet, do smaller sections) the mm or so that is sticking out should be easy to sand off, or you could use a flush trim router bit. i only did a single 15" speaker so it only had to be done once, but it woeks great. just make sure that the inner circle is still properly attached to the outside, otherwise your router may wander into the outer edge of the baffle

