I cut a few 1/2" wide long acrylic strips on the table saw, then cut them down to approximate length needed on the miter saw (not shown in video). I suggest cutting a shorter strip starting slightly wider than 1/2", test it to see if it will fit in the slits in your 2x4s, then trim the width down gradually on the table saw until you get the width so it fits the slits in your 2x4s snugly (e.g., so you have to use a rubber mallet to push them into place and they fit tightly). Once your width is dialed in cut longer strips.

Then take the long strips to the miter saw and cut them into smaller pieces. I found that 4.5 - 5" strips worked well. The length doesn't need to be exact. Just cut the strips to a length so a half inch or so sticks out on either side of the 2x4 when you insert them. Now use a dab of super glue to hold each acrylic strip in place in the 2x4 and insert all of them (using a mallet if needed). NOTE: It is really important you have a snug fit and use super glue. The acrylic strips need to be in the slits tightly so you can cut them flush with the 2x4s, and they don't fly out when you cut them.

I then used a circular saw to trim the acrylic strips flush. NOTE: THIS IS DANGEROUS, DON'T HOLD ME LIABLE IF YOU DO IT THIS WAY. A safer way would be using a table saw crosscut sled if you have one big enough. The safest way would be to use a flush cut hand saw to trim the acrylic flush. This is tedious, but I recommend this technique if you can't do it on the table saw. Be sure to wear eye protection too while doing this.