I'm looking for help on the method to create a structural support on this Solidworks model. I need to create a rectangular structural beam at 90 degrees but it appears the part won't combine the features into a finish. I'm looking for a recommendation on how to use the structural features to accomplish this. I've never used the structural features before so I'm trying to learn the best methods and welding approaches.

I'm trying to have miter cuts on the ends of this rectangular piece so it can be more easily welded onto the structure.

I'd like to remove everything in red for a nice angled cut that touches the leg with all portions of the extrusion but I'm not sure how to get this to work. It only extrudes to this point and I cannot get the portion above the centerpoint to extrude into the piece. I was thinking maybe I could extrude the top portion in a sketch afterwards, but I don't think the weld cut list will populate correctly.

Same for the top part of this structure

Any help is appreciated!