Installing a slightly smaller window than opening

[n8andm]

Trailer has opening at 33" wide, my window is 31" wide. I was planning on closing in on the opening, with additional aluminum (channel?)
Has anyone done this? Is there a thread for this already? Window is a good match to the one directly across from it, but right now there is a piece of plywood in there, no leaks though!

[68 TWind]

Have you tried finding a salvage replacement window? Might take a while but IMHO would be worth the effort.

[n8andm]

This IS my salvage replacement window, it's hard to find anything that comes close. And the fact that it looks like the one that should actually be there is a bonus. I do like to make things complicated- like buying such a project.