I need to install a Honeywell 10x10 damper into a 11x11 duct.
Options that I came up with:
build an adapter to reduce duct to 10x10 (alternatively I can use 12x12 damper and build adapter to enlarge duct to 12x12). (Honeywell dampers come in even sizes only).
Fill the gap with some sort of a frame made out of (circle one): metal, foam, wood.
I would go over sized rather than undersized if you have the room to do it. An undersized unit will limit your maximum flow, unless you always will be reducing the flow to that area then go undersized. Strips of ducting can be purchased at most home stores that you can rivet from one side to the other then use a good quality duct tape to seal. They make a tool to bend the sheet metal makes nice easy bends. Hope this helps.
A unit like that is designed to slide in from the side or bottom. I'd simply install it as normal, and add 1" metal flanges to the top and inner edges on either the upstream or downstream sides using rivets or screws. It's probably not important that the damper be perfectly centered.
You'll need to cut slots in the duct to accommodate the top one. The hole will look like this:
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I'd echo Ed's sentiment about oversizing, though. If you have space, consider adapting to a larger duct where the damper will be installed, and install a damper at least as large as the original duct (or slightly larger--the damper introduces resistance). Ideally, You'll get a 12" duct and a 12" damper and no modification will be necessary. Otherwise, install it using the technique I outlined above.
