> need to limit one of the traps in both of my front wall corners to approximately 12 inches wide. <
If you make a trap half as wide you need to make it twice as tall, or twice as deep, or a combination that results in the same internal volume [cubic feet]. But I think one foot is probably too narrow because the front panel won't be able to flex sufficiently.
> is there an easy rule to determine the typical frequency response a trap is tuned to based on size. <
According to Everest's book "The Master Handbook of Acoustics," if you use 1/4 inch plywood spaced with 4 inch wood [which is really closer to 3-3/4 inches] the formula is:
Center Frequency = 170 / sqr[0.74 * 3.75]
Where 0.74 is the surface density of 1/4 inch plywood and 3.75 is the depth of the chamber in inches. But I am suspicious of this formula because it does not include the height of the trap which affects its internal volume.
> do you sell your trap hanging devices seperate? <
No, but it's just a piece of wood cut at an angle!