Sorry about not getting back to this sooner. Work/ life got a little nuts.
1st lets start on the flanges.
Take your two 4" circles and tape them together (if you can't find them, they can be made simply with a circle cutter and a router. Even square would be fine, just be less attractive)
Now take a compass and draw a circle ~ 3/4" from the out side of the circle and 4 bisecting lines.
(They don't have to be perfect, mine are not. Its for me so I can live with it

) if they are not perfect, It is helpful to but a small "witness mark" on the circles so you can align them later.
Now drill them with a # 29 drill bit (if you do not wish to tap the acrylic as you would rather use wing nuts, up size the bit.)
Remove the tape and then redrill the holes so the screws fit freely. Now counter sink the holes. Go very slowly with the counter sink. If you have a drill press, use the slowest speed. With almost no t no pressure and watch for signs of heating (milky looking, smelling plastic etc)
Notice the center hole, I intently drilled it with too much pressure and speed. Notice how it chipped out? When you do this, and try to glue it to another piece of acrylic, it will craze (small cracks)
Now on the "top" flange, go ahead and drill a center hole 1/2", a 1/4" hole at a slight angle facing towards the center of the tube and tap the "lower" flange
Tube prep will be next.