The new Framemaker utility is going to save a lot of time in prototyping frames and other structures. It can create a frame which conforms to either a 3D wireframe (placing a frame element along each line) or you can wrap the frame around a shape, following its edges.
It does not create constraints between the frame members (it just locks their positions), but it does take care of cutting the ends to fit each other (miter, butt, etc.) and can "dress" the ends to prep for welding.
Framemaker creates the structures using profiles included in the Framemaker library. As delivered by Autodesk, these are standard ANSI, ISO, etc. tubes, angles, I-beams, etc. You can add custom profiles, but it is not a trivial task.
Sean Dotson has a tutorial up already on adding a custom size for an existing profile and is working on one for adding your own profile (I'm thinking 8020, Bosch, etc.).
While you're at Sean's site, make sure you check out the winning entry in the Tinkertoy Inventor modeling contest - it's amazing.