It has been suggested that I use 3m 4000 to bed the plexi into the frame and against the cabin. I also have considered using 3m Marine silicone.
Any comments or suggestions?
The 4000 would be fine for bedding the aluminum frame to the cabin wall. But it is not suitable for adhering the acrylic pane to the aluminum frame. Silicone isn't a good adhesive at all. Believe me, I have been though this many times on Mirages, Kirby's, Tanzers.
The Dow 795 is by far the best for the setting the panes into the frames. You likely would have to get that from a commercial window company. The other possibility is Sikaflex 295 which is stocked by marine stores. You could use the 795 or the 295 for setting the frames into the cabin, but the 4000 would be better if you ever want to get them off again!