The sizes quoted are in millimetres, so 16x16 or 3/4" X 3/4" would be just what you wanted. I looked on the Home Depot web site but I could not find any ducting whosoever, so I must assume they do not sell it on-line.
The way it works is you cut it to length, stick it on the flat surface like the skirting, put the cables in it and the lid snaps on, job done.
Its in shiny white PVC, so it matches in most cases.
Keith..