I picked up some black interlock for the shirt and some green corduroy for the pants. I figured as long as I was going to the trouble, I could go ahead and add some appliques to the outfit to 1) make it a little more special, 2) make it obvious who he was supposed to be, and 3) have a custom set ready for him to wear to Disneyworld next spring.
For the mock turtleneck, I used my Raglan Shirt pattern. I doubled the length of neckband (cut the piece on two folds rather than one), and even though the fabric had at least 100% stretch, I used the 75% cutting line just so I could be sure it would fit over his head. Otherwise, I put it together exactly as described in the pattern.
Doofenshmirtz doesn't get very far with out Agent P (Perry the Platypus) popping up, so I added him to the Easy-Fit pants.
We still need to work on hair and makeup, but he has the evil scientist look down pat!
Curse you, Perry the Platypus!