Be aware that a 5 inch prop is 128 mm across so any duct made will be 160 mm across. This includes the duct wall and the duct aero-lip. Below is a photo from my Excel spreadsheet. Obviously you can make one with a smaller lip but most of the lift would be lost.
Consider that a 5” set i had made from SLI Nylon was really $$$. Worse yet, the prints were about 40 grams each. Scale this up to a 6” design and the mass would easily be 60 grams per duct! Sure i could of made them lighter but they’d break from the slightest impact. The resulting duct-wall collision with the propeller would cause the propeller to implode. Thus the copter would fall and crash badly, wrecking the other Ducts in the process.
Now consider a 3” duct has about 8-15 grams and you’ll soon realize that this doesn’t scale up well with the copter size.
My conclusion: Prop-Guards are better than Ducts.
-Lighter in many cases, 50%
-Much cheeper to print.
-Can still reduce prop-tip vortices if designed right.