1) ventilation on the back - no proper airflow when inside an expedit cell
2) mainboard ports on the back - when you need to plug something you will have to draw the case out, and the cables cannot pass from the back to the wall sockets without cutting the cell.
3) No case exists that can utilize all the space inside the expedit cell, either too small, or just a little larger that 33cm...
So here it comes, it's my first try so more optimizations could be possible.
Step 1: Harvest the Parts, and Find a Configuration Best Suited.
and btw the Lekman Box
Step 2: Fix the Mainboard.
Use nylon spacers, (drill some holes if the spacers have attached screws - they are better, more stable ) and glue the spacers.
Mark where you will cut for the faceplate.
Step 3: The DVD (optional)
So later to install it you can simply slide it in, mark where the screws go from below, drill the bottom, fix it in place.
The orientation is your call, depending on your desk.
PS Check that it supports vertical mounting
Step 4: The Power Supply
BTW use a black one, and branded, I use here a lame bulk that came with the pc.
A note on safety, if your power supply catches fire... with all the wood and plastic around..., so don't leave the computer running unattended, for large periods, this is a desktop pc. And go for a power supply that not the cheapest one you can find, all power supplies say that they are "burn in" but, I don't trust anything that comes from China when it comes to fire safety, and I am not being racist, I am talking from own bitter personal experience.
Step 5: Cut the Front of the Lekman Box
the Jigsaw I used has a very thin and small blade that my sister uses to cut metal sheets in her jewelry making hobby. The blade brakes easily, cause it gets hot, melts the plastic and gets trapped.
The general guide lines are drill holes on the corners, pass the jigsaw blade through a hole, attach the blade, connect the dots. Cut less that you have marked, and use a rasp to perfect the slit.
This is my first try at cutting plastic so any comments or suggestions are welcome.