A couple of days ago, I needed a 4 inch to 2 1/2 inch hose adapter to connect a new piece of machinery to my dust collection system, which uses 4 inch plastic drain pipe. I thought I had one, but didn't. My local woodworking store was out of them, as expected.
Of course, I could have ordered one, but I needed to do the job right then, not 2 or 3 days later. I decided to look through junk around here and see if there was something I could cobble together to get me by.
My late wife saved EVERYTHING in case she ever needed it. There's still some stuff here that I hadn't gotten rid of yet, and one was a coffee can full of detergent jug lids. I took the can to where the hose was, and as luck would have it, a Gain Detergent jug lid was the perfect outside diameter to fit snugly into the inside of the 2 1/2" hose.
So I cut the end off the lid at the bandsaw. I also had some extra 4" blank pipe caps here. I drilled a 2 3/8" hole in the center of the pipe cap, and that allowed the threads on the jug lid to screw into the hole snugly. I still glued the connection for strength.
So I installed the hose clamps and mounted the homemade reducer onto the 4" opening that I had set up in the plastic pipe, and it worked just fine. Another thing I've done with detergent jug lids in the past is to make a temporary reducer to go from 2 1/2" dust hose to the small 1" or 1 1/4" hoses that come with some small power tools like sanders, etc.
If memory serves, a Tide Detergent lid will allow the 2 1/2" hose to be stuck into the inside of the threaded end of the lid. The threads on the lid mesh well enough with the spiral on the hose to stay on. The connection can still be taped around the outside for stability. Then you can drill a hole the appropriate diameter for the small tool hose to go through,then tape it in place. Not a permanent setup, but a quick fix when you're in a pinch.
Included are pictures of the 4 to 2 1/2 adapter. I don't know why some of them are rotated 90 degrees, but it's too late in the day to mess with it. You'll get the idea. By the way, in the last picture (wall mount.jpg), the spots on the wall that look like blood stains are from the last person that laughed at one of my homemade concoctions. Just sayin'.