Printed my duct in ASA-X at the time, printed the (straight) struts with a hole inside and epoxied 3mm cfk rods inside. Then added a layer of cfk only on the outside of the duct, easy to sand.
See here: Daniels (un)geared inflatable (slowly built) - #18 by JvdZ
Although I don’t use this duct anymore, it was solid enough to not deform under load, it worked quit well.