Maybe like that...
Make a Cylinder at the height wished
Boolean Union
Then Trim and modelise as you want your Cylindrical object

...and better to post your actual file 3dm with problem!

...else you can close an inclined circular surface with Construct / Nsided with Bulge = 0
(just trim the perimeter for have several edges continuous
and don't make the trim on the generator circle! )





etc...