How to join this Polysurface?

How can I join this open Polysurface together? I have “patched” the above and below opening and they are now open surfaces, but how do I join them to the open Polysurface?

Many thanks,
Robin (alias geniusupgrader)´

Here is the file:
Howtojointhispolysurface.3dm (554.6 KB)

Join works to join the smaller end surface to main surface.

Join does not work for your larger surface, probably the edges of it are not close enough to the edge of the main surface. Maximum gap is 0.033.

I created a new surface for the larger end using Patch and it joined without any problems. Check the parameters you are using in Patch.

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Ok, thank you, I will try it again. I overlooked the Options of the Patch Command.