Hi Spencer,
Lots of people use PVC as suggested but one problem I see with your design is securing the PVC towers to the tank. You probably don't want to permanently bond them to the bottom glass yet silicone will not provide a reliable bond between PVC and glass as you have drawn it. If you go the PVC route you will need to create a more stable base for the towers.
Another option is to build towers of light diffuser material (egg crate) as I have done in this photo:
The panels are held together with electrical zip ties and the normally white egg crate can be spray painted black using Krylon fusion paint.
To make a tower as in your drawing I might suggest zip tying three panels together measuring say 3"x12" each (just for example). This will give you a triangular tower 12" high. The tower can be secured to the bottom of the tank using aquarium safe silicone. This is done by running a bead of silicone on each side of the egg crate and then gooping it through the grating so that the silicone meets and globs together. When it dries it will hold the egg crate firmly in place.
Here is a photo of how that egg crate structure looks now:
