I am trying to print a cuboid which is 3cm tall, 6cm deep, and 25cm wide.

It will be supporting a static load that is not really that heavy but for a long term.

So of course I started by making a cube for it, setting all walls up to 5 and infill to 25% for strength but that was a lot of filament. So I tried poking holes in it with negative shapes, cones, spheres, etc and while they overall decreased the filament used even the largest ones had a big tradeoff where the filament used in the new walls really cut into any benefit from the lack of infill area.

I would've needed some very big holes in it.

I have ran into this before so I am wondering. Due to the nature of how FDM works does that mean that in simple shapes putting holes in them is not really going to decrease filament much?

If you wanted to dramatically decrease filament but still hold up a static load in roughly the original rectangular shape what would you do?