I used a piece of wood under my three pieces of material and then clamped them all down. The two pins that you see are only for alignment of the three sheets because I previously machined the perimeter of them all before hand. Once everything was clamped down, I removed the pins and then cut out the center.
 

I used my razor knife to cutout the hole in the tube which went pretty fast. The wood that you see in the tube are test pieces. I thought that if we used four 2" X 3" or four 2" X 2" equally spaced for the length of each tube, that would help stabilize things pretty well.
 

I needed the trim plate to conform to the tube so I used the edge of my work bench to help form it. I placed the trim plate like you see below, applied some pressure, moved my piece slightly and then repeated. Slowly the trim plate was like you see here.
 

The acrylic is acting like a spring here, which is keeping the trim plate from sitting on the tube correctly. I ended up opening up all the holes in the trim plate and window so everything would fit right. I used nuts to fasten all the screws with, which was easily accessed from the end of the tube. However, this won't be possible once all the tubes are assembled. I need to figure out another plan here.
 

I remembered that I had some rivet-nuts leftover from building my 33 Coupe years ago and sure enough, I had plenty of the #10 size that I needed. Rivet-nuts, or riv-nuts for short, have threads inside them and squash or deform like a pop-rivet does. All you do is drill the proper size hole for the riv-nut that you're using, then screw a riv-nut onto your installation tool,  then pull the handle. This deforms the body of the rivet-nut and locks it into place.
 

I used the thinner trim plate for all my riv-nuts, which worked out pretty good. However, I needed a way to remove all of the screws on the outside without the inner trim plate from falling. Notice the two arrows pointing to the those riv-nuts? These two riv-nuts will be used with some flat head screws that go through the tube to hold this trim plate in place so it doesn't fall. Problem solved!