A Snail.

Mini beasts.
An odd theme for fancy dress at Big Boy's school.  But hey...
Anyway, he decided he wanted to be a snail.  So I had to turn him into one.  Lacking the necessary magic wand, we opted for the less permanent cardboard and sellotape approach.

Items needed:
Backpack to attach shell to.
Old filing box (cardboard)
Paint.
Sellotape.  Lots and lots of it.

This, of course, is just a fraction of the full amount of tape needed...
(Oh, you don't have to use Sellotape, I'm sure other brands work just as well, it's just that's what we had...)
Cut out two almost circles (using the straight edges of the box to ensure they're not complete circles) from big bits of the box.
Cut out about a zillion strips of cardboard, which need to be scored so they fold better.
Stick these to one circle...

And then, of course, attach the other.
Cut a long strip of cardboard (I went against the grain, so to speak, so that it curled more easily).
Use vast amounts more tape to attach this to the struts between the two sides.
Find some paint (the same paint from the pirate ship we had at Big Boy's birthday...)
Paint the swirly bit.

Go to breakfast and ask people there how to attach the shell to the backpack.  Take their advice about punching holes in the shell and tying it on.
Leave breakfast (having not eaten yet...) and return to attach shell to backpack.  (You can just about see the string there...)

Put backpack on child.

Send child to school (well, take the backpack off first, otherwise it will get crushed in the car).
Hey presto.
Optional:
(Return from school run and have breakfast and shower, wondering why you didn't make the backpack the night before it was needed, rather than on the morning of mini-beasts day)

*************  Update ************* 

So, this afternoon, I go to collect Big Boy from school.  I'm not expecting the shell still to be in one piece.  It's not designed for endurance.  Well, strictly speaking, it was barely designed at all...  Anyway, out comes Big Boy carrying his book bag and his jacket, and with his shell slung over one shoulder.  I notice that the bottom bit isn't quite as stuck together as it was when I dropped him off this morning, but other than that, it's survived the rigours of a day in Reception.  Big Boy strolls over to me, unceremoniously dumping all his stuff at my feet.  He looks at me, and with a very pleased grin, says, "I didn't wear it all day".  Oh, well, isn't that wonderful.  I think this information is supposed to please me.  It doesn't have the desired effect.  I'm just left thinking that I could much more easily have blown up a brown balloon and taped it to his back.  Imagine the time I'd have saved.  Oh well, on the bright side, any fancy dress party for the next three years, I shall force him to wear his snail shell.

And to add insult to injury, tonight's prayer went thus:
Dear our Jesus (I've tried to point out that 'Lord' is more common than 'our', but he's not taking the advice at the moment, and I figure that 'our' is theologically ok) thank you for the fun I had today playing bees and wasps and ladybirds and butterflies... at this point, I add a hopeful, 'and snails?'.  To which he immediately responds, 'no, I was a wasp.' 
Parenting rocks.