FYI- most sheet goods are now metric. They show it as inches for the full sheet dimensions but often, the thickness is really 19mm.
BTW- having built literally hundreds of boxes for speakers and mostly for subwoofers, I highly recommend using some kind of mechanical fastener to secure the sides/back/front/bracing, etc in addition to the glue. I know the glue holds very well, but the face of MDF isn't as porous as the edges or real wood and it won't absorb the glue in the same way, so it doesn't hold as well. Narrow or box staples work very well, as do drywall screws. Any excessive depth in the holes when using screws can be filled- I prefer Bondo auto body filler, but a good wood filler will work, too.
Remember, nobody has too many clamps.
Also, and this is for when you build other boxes and speakers, I see that the sides appear to have rabbets at the front, to accept the baffle- if you used a router for that, I would recommend routing a dado for the edges of the bracing because you'll have no chance of a glue joint failing and it actually makes assembly faster because the bracing can't slide on the other pieces.