If I understand you correctly that your windows are 25" wide, then you
should not need a casement unit- a regular one should fit. The casement
units serve a specific purpose (limited market), and thus are harder to
find, only made in a couple of capacities, and are much more expensive.
If for some reason you want/need to use a casement unit, you can just
make your own filler panels out of plywood, masonite, or whatever. Those
plastic sliding fillers are pretty flimsy anyway, and usually are pretty
well deterioatedafter a couple of years. Good luck Larry