I'm looking at my stuff, and I'm not seeing the problem.
I have 4 different gravel vacs (Whenever I buy a used tank + equipment, seems their is always a gravel vac). All of them would fit the python- Pop the clear, flexible plastic tube off the green cap of the python, and pop it on the same part of the other vac. Maybe it's just chance that mine seem to fit? You could maybe use a hose clamp if it is close?
Another idea might be to increase the suction. If you are using an inside sink to power your python, you will almost certainly get more power using an outside faucet, like a garden hose. If you have a multi-story house, you can also get more power by using a sink on the floor beneath you.
I have enough gap in my ductwork to snake the python down to my basement sink. Also, if you have a problem with large stuff (For me, java moss) clogging the python, you can disconnect the faucet side after starting the siphon- In this setup, I find I get more suction just from gravity then I do hooked up to an inside sink on the same floor.