We need something like the fuel tanks, with the same reshaping features, but it's hollow instead of full of fuel. You could make engine skirt with it, space station trusses, cargo bays and lightweight sections of craft with that. You can style it to have a solid panel outside or girders as in the interstage.
The interstage is not this. This wouldn't separate from the vessel and would have both ends open. The cargo bay is not this, because this doesn't have doors and it can be styled differently.
I ended up using a closed cargo bay to scoop up a stranded drood in capsule but found that unless I turned off collisions on the entire bay nothing could enter the "open" end of the bay. I had a dock port in the bay that docked with the capsule and the point of the bay was to protect the capsule (no heat shield, doh) during re-entry. Anyway, agreed, need a hollow open-ended general tube part