All you need are two pieces of felt, some floss, and a sewing sharp.
This part will depend on the size of felt you're using and the size of the magnetic board you have. My felt is 9 x 12 and my board is 8 x 11. I laid my board onto the felt and aligned it so that I had a 1/2" border on every side.
I used pins to mark the 1/2" border on three sides. I left the right side open for the board to slip in, but you could leave any one side open. Set this piece aside.
The border on the second piece of felt should be about an inch.
Get a piece of copy paper from your printer (8.5" x 11") and trim it so that you have a paper template that leaves a 1" border on each side of the felt.
Pin the paper template to the felt and cut around it.
Place the "frame" on the first piece of felt, secure at the corners with pins, and then sew or stitch (or glue, if you want to) the two pieces together.
And here's my chart protector. My magnetic board with the chart on it slips into the right side and the edges don't see my hands again until I get ready to move the chart on the board, which--let's be honest--will be a while!
So there you go! An easy felt chart protector for those gigantic charts.