For the insert that goes in the slot in the top of the cylinder, there is not need to create a new model, since you already have the profile you need in the cylinder block.
Just make a copy of the cylinder block, draw an outline for a cut, make sure the "cut" goes to the outside, and presto, you have your insert.
Always make use of any existing geometry you have to make a new part.
Don't reinvent the wheel if you don't have to (think smarter, not harder).