I did another try here
2 small rectangles at the end a bigger one in the middle which the rails goes into the middle of it...sweep then move this whole solid into my model pushing the other half into my model.
That seems to work but probably not the best or efficient way to do it?

Michael what you shown me was extremely useful, the reason I wanted for a 2mm thickness so it could be easier to sand when fitting the 3d printed item.