Product review (1 stars): Instead use 6 inch foil duct as a replacement (see picture)
This duct is replacement for the OEM part included with the portable AC unit from LG mode #lp0910wnry2.  This duct is terrible value: expensive and very short life (made of cheap vinyl) and, with any amount of regular use, it will crack and disintegrate before your eyes.... rendering your cooling unit T.U.  because it cannot exhaust the warm air out of your window slat without the duct.  This duct is about 5.75" O.D. which fits INSIDE the AC unit flanges.  Alas, there is NO generic 5.75" replacement at your local hardware store.  HOWEVER, look for standard foil flexible duct with 6" O.D. (at any length >72" should work) and fit this the foil duct over the OUTSIDE of the stock flanges and secure it with plumbing clamps.  Problem solved and bulletproof compared to the crappy OEM vinyl material duct.