Finding solution (any help will be appreciated)

Hi,
we have a problem with modeling our object. The problem is that we can't place a generic face based object on a right place in model.

we made it like that, by sinking model down to have this small area (in the hole) who would stick, but then we notice that object should be sticking on this red line that I drew (small angled line on first picture)..to be a pefectly fit..so we rotate it like this:

and tried to insert in the project again:

whatsoever, it is not really going on the right side, no matter how hard we tried (tabs and so on..)

we tried also this way, so this bigger area is sticking..but is not going inside also.:

we are interested if there is some way to do that in Revit? :))

Thank you!

Polona e Karla