I hate to disagree with mfpmax, but I think this will work fine. In the past, I've simply press-fitted nylon hose together for makeshift reducers (selection at local hardware stores is pretty weak...). I was amazed at how well 5/8ID|7/8OD hose stayed inside 3/4ID|1"OD hose. I didn't even use adhesive, I just rounded the end of the hose a little with a knife, wetted both hoses with water, and press fit them together for 3/4" overlap. No matter how hard I tried, I couldn't pull them back apart (the hoses just stretched, and I got tired

)
I realize that two vinyl hoses are going to stick together better than copper and Tygon, but the goop should fix everything, IMO.
Just make sure you pressure test it for a while, just in case.
[correction]BTW, that mount for _mashie's_ eheim looks great. I think I'm going to have to steal that idea.