Step 5: Back panels & filler
I measured the channels on each block and copied it onto some 3mm MDF using squares and suchlike. Get it as square and accurate as possible as the back panels will hold it in shape.
3mm MDF isn't the toughest material in the world so with a bit of persistence I got through it with a knife & a straight edge.
Then I glued them in place, wiping the glue off the inside
Once that was dry I rubbed in a good amount of filler, and it all got a good sanding (use a dust mask).