replying to KenK, Iggy wrote:
I know they're fairly small holes, but you'd be surprised and find you really
need rigid stuff. So, stay away from Aluminum, it's way too soft and weak.
You'll need Steel, Cast Iron or Wrought Iron. A Metal Scrap Yard or Ornamental
Iron Store/Shop would be your best bet for 5-6" X 11-12". Those places will
usually cut-out what you want and it may not be much more to have them bevel
and round everything (you don't want knife edges or corners).

However, to give you an idea of what you might be looking for. Home Depot has
pre-cut 6" X 18" 16-gauge (the lower the gauge the better and thicker) sheets
at pretty high prices of $7-$10 each. See:
http://www.homedepot.com/p/Everbilt-...1467/204225705
OR
http://www.homedepot.com/p/M-D-Build...6078/205058576
OR their best and cheapest option of 6" X 24"
http://www.homedepot.com/p/Everbilt-...0687/204325597

On a couple of final thoughts, you may also want to consider getting some Batt
or Foam Insulation to either glue to the metal or to loosely stuff into the
vent holes. To keep out hot and cold temperatures as well as any possible
bugs. I'd also countersink drill and screw the plates into the floor and paint
the metal so it doesn't try to rust on you anytime soon.

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