You could simply order the right(ish) sized piece and drill the holes yourself...
-Take your side panel now, and draw (inside) how much of it you could cut out, leaving room (say 1/2 an inch) for rivets.
-Then, get a piece of acrylic that is approximately 1/2 inch larger than the hole (so it overlaps where the rivets would go).
-Drill holes for the rivets to hold the acrylic to the case side
-Mark out where the fan holes will go (you can just hold the fan against it and mark)
-Drill fan holes
Viola, you have an acrylic side panel with a fan mounted on it. You could also consider (for a clean look) putting silicone along the edge of the panel and sheet so there are no gaps.