I have asked this question before but I am not sure that I am getting across what I want to ask. So here goes...

I have a tool base as a component in an assembly. The tool base varies in size from 3"x20" to 8"x30". I then insert another component "top plate" and that component needs to be 9" wide by the length of the x dimension of the tool base. What I do is insert the component "top plate", mate it to the base then edit the sketch for the boss and create relations so that the top plate component lines up as desired. Doing this manually every time is tedious.

Can anyone see a way to make the top plate component automatically mate itself and adapt to the correct size without me creating the mates manually and editing the sketch manually? Seems like SW is smart enough to do this somehow...

Attached is an example assembly.