[Floyd R Turbo]

That is all fine, the only disadvantage to a narrow screen is less turnover of the water passing over the screen, which mean slightly less effective filtering. But only slightly and this only matters, really, if you feed to the maximum. You can compensate for this if it becomes an issue by widening the slot and increasing the flow. But in 99.9% of the case, it is a non-issue because the algae takes the excess nutrients out of the water continuously and quickly.

I like your idea of the tapered box. This should work, but you probably won't be able to use a bulkhead type setup. The other concern is that if you get very thick growth, the growth toward the bottom may partially block the flow. This would lead to some good 3D growth, but could potentially block enough flow to overflow the box. What I would do in this case is make the box about 1 inch longer that the screen on each side so that when the algae does grow in and block the flow (and it will, if you make it narrow enough) then the flow has a place to go. This is basically how my box runs currently, I have holes in the corners of the box, which is 1" longer on each dimension. The growth regularly builds up and traps about 4" of water.

Also the tapered box would probably have to have the screen terminate in a sump chamber.