Hi Matadem, Trim is a lower level operation that only works at the surface level. It doesn't automatically glue pieces together like the booleans do, you need to follow it up with using Edit > Join to glue things together.

You also have extra pieces that you need to delete and should not be joined in.

For example in this area here:



Select this piece and hide it:



Then select this left over fragment and delete it:



Now show the box piece and select it and the main piece and use Edit > Join to glue them together.

Booleans can save a lot of time since they automatically throw out pieces and join stuff together. It would probably be good to see the step where you were trying to boolean it so I could give some tips on why that wasn't working well. Stuff like having surfaces just barely skimming over coincident surface areas can be difficult to boolean for example. You instead want to have things push through each other with a little margin rather than barely skimming unless they are very accurate like with planes directly on each other and not stuff like sweeps skimming over each other.

- Michael