HI,
This is my first post here.
If the vent holes in your cabinet are like the ones in the Gallery photos it's really pretty easy to do with a curve that large.
I like to use a piece of rough sand paper and rough up the fagric side (back) to break down the backing fabric letting it stretch around the inside of the curve. It will indeed pucker into pleates on the back side but those can be trimmed. This is one area where I use DAP contact instead of hide or water based adhesive because it helps the Tolex to stretch tight around the curved service.
Here are a few pics of similar curved areas.
EDIT: The first 9 photos show a single panel with both an inside and an outside curve, both are done as described above. You will only need to do the inside which is a bit easier than a good outside curve. You should get some scrap wood and practice corners and stuff like this before you commit to your actual work piece. Many of the photos are of my own practice pieces.
http://s238.photobucket.com/albums/ff24 ... d%20Areas/Good luck to you.
If this is your first Tolex project you might find some of this useful, or maybe not.
http://www.guitarkitbuilder.com/content ... er-cabinet