For the internal part cut one of your loops so it basically makes it into a rectangle. As far as the bottom piece I would connect it to some of the other parts but it looks fine the way it is if you don't want to.
The model itself is not that complicated, so it should be fairly easy thing to do. One thing to keep in mind when doing uv's is that they all function like unwrapped box. If you know how to make a box from paper it is essentially like that for all uv's.
As for the problem you are having, make planar projections from the all sides of the model, and triangle part too (top, and other sides of it) and than weld uv's vertices together. After you do that you can try relaxing entire uv island that you created.
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As for the problem you are having, make planar projections from the all sides of the model, and triangle part too (top, and other sides of it) and than weld uv's vertices together. After you do that you can try relaxing entire uv island that you created.
Bonus points if you can see what's not optimal about the above unwrap. Excluding packing / edge padding.