I ordered a $6 document holder from Aircraft Spruce that showed up today and it's a nice fit for the registration card but is very cheaply made from a piece of paper card stock with the plastic window and some edging sewn onto it.
I had previously purchased a $1 pencil holder from Walmart that's fabric with a zipper and clear window but it's a little large to fit where I wanted to put it. After seeing the quality of the purpose made holder I decided to give the pencil holder a try, so I folded the bottom over to the height I need and secured it with some high strength double sided tape which seems like it will hold it well. Added a few pieces of velcro to the back and stuck it in place. Another advantage of this holder is it's large enough to hold a full piece of paper folded in half so I can keep the weight and balance and operating limitations in it as well as the registration and airworthiness.