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
It also has droves where the 3 triangle support burrs will go.
Step 3: The triangle support
Each support has a burr that goes in the backboard and a burr that goes into the shelve. Obviously all must match.
On the second picture you can see a completed shelve.
Step 4: The Whole Thing
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