Hi Frank,

Calculate the area of the supply duct then cut the same area from the
bottom of the door. for instance if you have a six inch internal
diameter duct feeding your office the supply area would be 28.26
square inches so if your 30" door has an inch gap above finish floor
you should have enough return area. an eight inch diameter duct would
require about 1-3/4" above finished floor level. If you cannot get
enough by doing that put a return grill in your door. a lot simpler
than putting in a return duct.

This was my first idea, but I use the office to work in and I have to keep
the noise of a toddler out of it, so I can't cut the door or put a door vent
in.

Any other ideas?

Thanks,

Alan