The inside of the back and side walls were clad in 11 mm thick OS Board (a.k.a. Sterling Board).  This was glued to the polystyrene foam insulation with solvent-free builders' grab adhesive.  Over the entire project, I ended up using 19 x 330 ml tubes of this adhesive!  The edges of the panels were finished off with battens screwed and glued to the wood frame uprights and single small brackets to and bottom.

The floor and pier were covered with an old bedroom carpet.  The carpet around the pier was glued in place with solvent-free builders' grab adhesive.  In the picture on the left you can see the pipe insulation stuck between pier and the floor to prevent creepy-crawlies getting in.