When working with shell type parts, the shell operation sequence is critical. Too late and it doesn't work (part is too complex), too early makes the feature addition harder.
Addition (or substraction) of features/bosses on a molded component necessitates that you model the external enveloppe first, add it to the shell, trim it along the shell profile and then, substract the internal geometry to make this boss a shell of comparable thickness that is in continuity with the parent part. This is tedius. In CATIA, there is the possibility to make a union-trim that allows you to add a body while trimming the portion of the body that protrudes on the other sides. This helps a little but still the substraction of an internal body is necessary.
I suggest an operation that would allow to select the portion of the union you want to keep (a little bit like in the sketches for which we no longer need to trim lines) and ask for a thickness (internal or external).
