I like the angle stock reinforcement in the one picture, but then I tend to over-build on strength. I plan on joining plexiglas junctions with clear RTV / Silastic / Silicone Rubber, call it what you want, adhesive. Besides being a good adhesive, the natural deadening effect of the soft rubber will help dampen vibration in the larger flat surfaces. I'm not too concerned over whether it is "Pretty" as I'm not into "Window Dressing" my PC.
As far as a forcing cone goes, I'm not too keen on the idea because it imparts additional resistance to the airflow and axial fans are notorious for not tolerating resistance to airflow.
As far as having too great of a distance from the fan to the radiator goes, I don't believe there is such a thing as long as the channel is relatively airtight.
I have worked with both Lexan and Plexiglas and so far have come up on the lucky end of the equation. Depending upon the size of the sheet I am working with, on larger sheets I use my table saw to cut it. Just pay attention to the sharpness of the blade and your feed rate. You don't want to rush the cut and wind up melting the stuff. Then it gets as ornery as hot pizza cheese. For smaller sheets, I score both sides and break it on a sharp edged, flat surface. Most hardware stores that cut glass for windows can also cut you Lexan or Plexiglas for you. They usually have a neat score and break machine. If they happen to sell the stuff, buy it from them. That way, if they hose it up, its their problem.
Hoot