Lighting gels are 2'x2' colored, transparent, flexible, heat resistant, cellophane like plastic sheets that are placed in front of lights for color correction or mood lighting. They are sold in hundreds of colors at high end camera stores and movie supply houses.
Buy either the proper color sheet or a $5 sample book and cut out, with household scissors, a 1"x1" piece and tape it around the edges in front of the IR / LED window with gaffers tape. It is the cheapest, quickest, and least invasive way of solving your issue.
I use lighting gels all the time to darken the RF patch on my old RF cameras. I have several orange, amber, and yellow sheets cut up into various parts now.
If you are in L.A., you an buy a book of samples at the checkout station at Filmtools in Burbank for $1. At least you could a couple of years ago.
Try this from adorama for less than $3. You'll find something in here that will work perfectly.
Rosco Cinegel 1.5" X 3.25" Swatchbook 950SBCNG0103