Replacing In-Wall AC - Dimensions Question

[Mark Lieb]

I'm replacing Friedrich WS10C30-A. It's discontinued.
W (26.5") D (22") H (15.75")
Plug type 6-15P

My question is about sizing and sleeve. I found replacement in-wall AC but they are all smaller than the existing AC dimensions. This is my first in-wall unit. How do I replace it and make it look professional (eg no cardboard/Styrofoam)? All advice appreciated.

Here is a link to the old unit: Old Friedrich Unit