So, I thought if someone else wanted to make one I would post this tutorial. All info below the cut!
EDIT: I do not recommend using cardboard anymore as for some reason some of the pins won't hold after being removed. It is a one-time use thing. I am going to try to find a better solution.
You will need:
-A photo frame of your choice (I got mine from walmart for 3 bucks)
-Cardboard (one time use) or possibly cork board (I used a flattened box from the shipping section)
-Felt- There is some that has adhesive backing which makes it a lot easier
-Exacto blade or box cutter
Now onto the tutorial:
1. Take the frame apart into the 3 peices, the frame itself, the backing, and the glass. You can discard the glass, you do not need it, and it won't fit over the pins once setup (unless you have a shadow box)
2. Using the backing as a guide, cut the cardboard to the same size. Or you could measure with a ruler, but I was lazy. Also, corkboard would probably be a good (or better) material if you can find it, but I have not tried it myself.
Don't worry about the edges being perfectly straight, they will be hidden by the frame.
3. I also used the frame backing as a guide for cutting the felt. If you have felt with adhesive backing it is best to use an exacto or box cutter, it is near impossible to cut with scissors. Same as with the cardboard peice, the edges do not have to be perfectly straight either.
4. Stick the felt onto the cardboard, then assemble the frame like normal except with the felt cardboard in front of the backing instead of the glass.
And stick your pins on and done! The cardboard seems to hold them pretty well, but they had to be pushed in as far as they could go.
Thanks for reading and hope this short guide was helpful :)