Thank you all for all the good advice.
I was able to speak to a tech rep for Pelican and he said to purchase an electric knife that cuts foam... I picked one up on Ebay for $30.
I also used a hot knife as mentioned above. I traced an outline on cardboard then shaved the dimensions down on the cardboard so it'd fit somewhat tight. Be sure you have achieved the final set up of your gun.
Go to a good hobby shop (not a hobby lobby or anything like that) and get a foam cutting blade for a exacto knife. They are long and will do the trick. Not expensive either. About 3-4" long.