Aspenite is not very robust and considering it's only 5mm thick you know this wouldn't make stable shelves easily.
So I built triangle support. I'll show you in this instructable.
Step 1: Back Board
So I came up with a board like shown on the picture.
(Note: it's possible to first create the board and cut the droves, but it's better actually to first build the shelves and then dig the droves accordingly to where their burrs come.)
Step 2: A Shelve
I built four shelves like the one on the picture. You see it has mortises that will go into the back board.
It also has tenons where the 3 triangle support burrs will go.
Step 3: The Triangle Support
Here's what I put under each shelves - three of these triangle supports, which now will make it stable.
Each support has a mortise that goes in the backboard and a mortise that goes into the shelve. Obviously all must match.
On the second picture you can see a completed shelve.
Step 4: The Whole Thing
And here is the whole thing assembled. While the droves and burrs do most of the work you still need to use some glue to attach the parts together. I even used small nails put from the back board to strengthen the construction further.
At the end the backboard is attached to the beams in the shed using metal plates. If your shed has wooden walls you can use simply some screws.
I have other projects like this on shedbuilder.info