Here's a quick tutorial and demo of how I created some invites back in 2012.
Step 1: Pie Lids
So the first thing is to order the foil pie dishes. You can buy these from catering suppy stores and cake shops.
The goog thing about this is you can use this same idea for a scary candy or sweet dish for guests to open. To create the pie eye topping, I took a picture of the top of a pork pie and then a photo of an eye and then used photoshop to comp the two together make sure you use shadowing internally and around the eye to make it realistic.
Step 2: Pie Filling
I chose to use some cheap Halloween sweets like eyes and noses as these are the Halloween Sweeny Todd pies that Mrs Lovett makes in the book. Just make sure you measure your pie lid to fit so that the foil seals around the lid.
Step 3: Box
Next you need to make a box. There are loads on the web and on Instructables so just find out that works and create your cutter. I screen printed mine but if you can't be bothered I sell these in my Etsy shop as a downloadable file that you can resize depending on your printer.
Step 4: Assembly
The last thing to do is to assemble the box and either just tuck the flap in once the box is filled and packed out or create a fastening as I have above.