One possibility for you would be to find a small, but competent, sheet metal shop that fabricates HVAC duct work components. If you find the right guy, he can fabricate a metal duct adapter that can fasten to your existing deck and permit the transition to a round flexible hose like used in the project you have been looking at (your URL). You would need to leave your deck with him, so if you do go this route, make sure your deck is as clean and free of clippings and matted gunk as you can get it. Otherwise, if there are any shops nearby that repair OPE, you might visit them to see if they have any "derilect) tractors around back that might have the type of snorkel you are seeking. If you decide to go the sheet metal route, be sure you have a few pictures of what the object adapter looks like so you can convey your idea to the sheet metal guy. He probably has no idea what it should look like but if he can see some pics, it ought to pique his interest (and give him a challenge). Best of luck and please post back with pics too as your project develops.