The first silencer is starting to take shape. I created the middle partition gluing and screwing two MDF fins into place. Along with two smaller pieces of MDF at the top which blocks in the side channels where the air exits the box. I wasn't sure exactly how I was going to construct the internal baffles but with a bit of experimenting I decided to use rectangles of MDF as the base and stretch the material over and pin underneath. Again, with a bit of practice I came up with a fairly decent method and the baffles fit very snugly into the middle section of the silencer. I'll need to make a similar version for the other corner but I thought I would finish this one first to iron out any problem areas. So far this seems to go together pretty well. I'll have to assemble the box in situ as I'll be unable to secure it to the sides as there will be limited space to access with a screwdriver. This is probably for the best as the finished box will be quite heavy if fully assembled so I'll be able to fit this comfortably without any help.
So to assemble the box in situ:
Attach sides to the wall allowing 18.5mm for the back to slide up
Make the main body with back piece and internal fins to create the channel along with the bottom attached
Screw the top to the ceiling
Slide the body assembly up the back and screw in situ
Glue and screw the bottom to the sides