Cut a piece of vinyl so it's around 1/2" to 1" larger than the face of your RGB matrix on ALL sides (so, 1-2" total extra width).
You can use clear or colored vinyl, or if you're fancy like me, shiny rainbow iridescent unicorn vinyl.
Fold back your pocket flap and mark the width of your RGB matrix with pins. We'll make a slit about 1-2 inches below where the top of the panel will rest.
Use scissors or a utility knife to carefully slice through all the layers of fabric EXCEPT the outermost layer -- leave that layer intact for now. This bag has lots of layers to cut through. For your bag you may only be cutting through one layer of lining.
Slide your assembled display into the pocket you just made and zip the pocket closed. Make sure everything fits with the USB cable connected and the battery attached.
Carefully cut around the face of the panel with sharp fabric scissors.
Depending on your bag's material, you may start to get raveling along the edges. Grab your bottle of Fray Check to nip that in the bud. Apply the Fray Check to any cut edges that look like they might fray -- I put it on the innermost lining and on the outer fabric just to be safe.
Be generous but tidy. We're going to put some trim over this edge later so don't worry if it looks a bit messy.
Place a sheet of scrap paper into your pocket to catch any extra glue drips. Then set the vinyl beneath the outer layer. Add some glue along the seam allowance of the vinyl and carefully press it down.
If you get glue on the vinyl, it's easy to get it off with 99% alcohol and a q-tip/cotton swab. Best to do this fast before the glue dries completely.
Let the glue dry overnight. Come back in the morning and check to be sure all the edges are tightly attached. Once you're happy with the seal, add another layer of E6000 along the outside and glue down your ribbon or trim to hide the raw edge.
Slip your RGB panel inside and see how it fits. My pocket flap is a bit wider than the panel and it was sliding to the side. I want it to stay centered on the window, so I added a couple of stitches with heavy duty thread through the trim, about 2 inches in from each corner. The panel is still easy to remove but now it won't slip to the side.
You can reach inside the top flap to access the control buttons on the top of the Matrix Portal board. If you're using the on/off switch, plug the switch extension between the battery and the USB C cable and you can reach it here too.
Optionally you can edge-finish the raw edges of the pocket with a hand stitch or bias tape, or more sparkly ribbon or trim.