Uh - you need 2 or 4 negative forms. And the crossing? Interesting - quite challenging for the manufacturers methinks.
Now that's both an elegant and pretty structure, which is what the very best of real world products possess. Very nice!For the downtube, fill the bottom of the headtube first,
then select polygons where the connection will be.
Then use Cover to create new geometry and pull straight out just past the edge, scale flat, cover again
and pull out some more. Then rotate from the top corner into position over the Bottom Bracket.
This is what we're going for: