Step 1: Safety
as such it could kill you, cause damage or injuries. If you are not really skilled at mains
powered electric device and related safety building practice and are not well aware of the risk related, you are suggested to have a friend help you with this project.
Also, as a general rule, when you are working on dangerous things always have someone next to you instructed on what to do if something goes unexpectedly.
The main issue here is to make sure that in no way metallic parts of the prongs and shapes come into contact with the electric heaters of the toaster.
These notes are not just to scare or bother anyone, but I absolutely want that fun does not turn
Step 2: Parts
edit: thanks drcrash - Aluminum "flashing" from the roofing department of a home improvement store (1 mm thick circa)
A paper template
Scissors (or a LASER cutter, if you have one...)
Dremel or similar (not necessary, depends on shape's complexity)
A small fine file
A hammer and a wood block