The flanges do fit very tightly in the 4" aluminum tube. But you shouldn’t have to go to that much trouble to get them installed. It helps if you use the hand vacuum pump to create a vacuum in the tube. It’s kind of a three-handed operation at first, you have to have the flanges pressed against the tube with enough force that they’ll seal- then attach the pump to the enclosure vent port, and pump the internal pressure down as far as you can. With this internal vacuum, the flanges will snap into place with a modest push. Just keep your fingers out of the way of the flange! When it snaps into place it will pinch anything in its way.
Things are more interesting when removing the flanges from the tube. There are notches in the flange that can be used to pry the tube off, but the aluminum is very soft. I use thin, flat blade screwdrivers, with the flat blade covered with some adhesive-lined heatshrink. The heatshrink serves as padding for the aluminum, the adhesive lining keeps it from twisting off of the screwdriver. This works well for screwdrivers whose blades are 0.8-1.0mm thick.