Why don't you just buy a rubber adapter instead ? The best one, in my opinion, is the kind that fits permanently in the bottom of the bucher funnel. When you place the funnel atop the filtering flask it forms a tight seal.
Another kind is a flat rubber disk, with a hole with conic edges, just place it atop the filtering flask and the bucher fits exactly over the hole, also forms a good seal.
I have two different sized bucher funnel, one kind for each, both work fine.