Took this to the London MCM Expo in May and got a few lovely comments on it.
The box itself is cardboard covered in tartan fabric and its lined with red fabric. In the bottom it has a secret compartment where I kept my money, phone etc. The outside has black lace, 2 black straps, 'eyelets', a chain, shrink plastic charms and a heart patch made using iron on printer paper. The lid is secured using velcro :)
I also made all 4 of Amu's eggs, but only 3 fit :( If I'd made it big enough for 4 it would've been HUGE! The bag is worn using a belt.
The second from last image (8?) is the reference I used from the anime ^^
EDIT: New picture added! The eggs which the bag holds :)
EDIT 2!: Hey guys! You can now see my Amu cosplay on Cosplay Island! :) http://www.cosplayisland.co.uk/costume/view/17162 Minus jacket and box sadly.. I didn't have room in my suitcase :(
So first cut your box down to size (or make your box ^^) I had already made my eggs at this point, so I used them as a rough guide as well as the items I wanted to hide inside.
Next cut the thin piece of card to the shape and size required for the false bottom (if you want one!) Then cut a rectangle of this card to make a hinge from.
Fold the hinge piece in half, then use it to hold the false bottom in place (pic is easier to understand)
Line the inside of the box down to the false bottom. You don't need to line under it cos no one will see it! Don't forget to cover the inside of the lid!
Fold the edges over to the front, you won't see them when the tartan is over the top.
Lay the lace around the edges of the box, the corners are a little tricky, so take your time.
Cover the box in the red tartan fabric, use the double sided tape to secure it :) (don't fold the edges of the fabric over the inside of the flap, or you won't be able to get your lace in!!)
This step took the longest for me cos I ran out of tape XD
Cut your black fabric (belt!!) to size (leave excess, it needs to be tucked in)
Secure this down to the front of the box, then peel back the tartan a little to poke the edges in for a nice finish.
Round off the bottom of the vertical strip.
If you're using actual eyelets this goes before step 8. Punch the eyelets into the black fabric strips, equal distances apart.
If you're using things that look like eyelets, glue these onto the fabric.
Print out the heart and crown design onto the black transfer paper. Iron this onto the thin black fabric using the instructions that came with the paper (most are different). Cut around this in rounds/ humps through 2 pieces.
Attach these 2 pieces together in the centre, and to the box by the centre only. This allows the edges to stand out and look all frilly :)
Cut a piece of chain to the correct length.
Use small pieces of wire to attach the chain to the front of the box.
Make the 3 shrink plastic charms.
Use jewelery findings to join these to the chain.
Attach velcro to the inside of the lid. (You may want to use super glue!)
Finally make a loop out of red ribbon and glue it under the false bottom, sticking up so you can pull it to lift the bottom up.
Carefully use a Stanley or sharp craft knife to make slots in the sides of the box to thread your belt through.
And now you should be done! Well done for sticking with it ^^