you can find clear pvc
online much cheaper. i like the look of the plexiglass tho because its flat and it doesnt distort the fish through the glass. one way to fill it is drill a hole through a cork and put an airline tube through it and stick it in the bottom of you bridge and just suck the air out. as the water rises the cork will float up into your bridge and in between breaths kink the airline hose. its sounds more complicated then it really is. i hope to do this within the next year but idk if its gonna happen.
look at this site for the pvc