I've found some Schneppers from Wipperfeld, district of Wipperfürth. This is exactly the place I'll be researching. What a funny coincidence. The problem will be to find the marriage as marriages to almost 100 % took place where the bride was from. And looking at the area I still can't find a place that reads or sounds like Slumborn.
But there were Spicher people in Wipperfeld also. So I'll try to find the marriage there first.
I do hope I can spare some time if they were not from Wipperfeld.