Here's a few photo's how I made mine.

Photo 1. Measure out where and how much you want to cut out.
Photo's 2, and 3. Install the chassis in the jig and then clamp it down.
Photo 4. I make the rear cut first, it seems to work better that way for me.
Photo 5. Make the front cut.

Then glue the front and rear together with CA.
I install the vertical carbon fiber braces.

Total time 1hr for the 1st one, (had to make the jig).
After that maybe 15min.’s ea.

P.S. I would have posted more pic's, but for some reason this website only let's you post (5 pic's per post)????