Maybe re use some parts for the space station, like they were thinking about for the current ISS. Like the shuttle fuel tank, converted to crew quarters etc. String a few together and you get a lot of volume to work with.
I like the instruments clustered to one side instead of blocking the view of the rocket in the middle. Looks cobbled together like a kerbal would, from spare parts.
It heats up after you dump the boosters because that's where your winglets are. The winglets act as cooling fins. Maybe move them to the 3m fuel tank and see if this helps? Needs MOAR boosters!