See if you have a TAP plastic store near you. They might be able to sell you only what you need. Also, you don't have to fill the entire space from top to bottom. Using standardized sizes always makes things cheaper. Look into project sheets. This is one of the few items that are significantly cheaper to buy from a place that routinely brings in pallets of the stuff, most of the cost of acrylic is the moving it around part, not the materials.
I like #4. You could do something like I did on my reservoirs and use 1/4" rubber top hat grommets and some elbows with clear vinyl tubing. You can see a more precise measurement with that method. I got the idea from the brewery I work at with their fermenters. I used a 1 gallon jug and poured it in one at a time and took a Sharpie to the tubing to indicate 1g increments.