I would assume that the 1st things you will want to do is figure out a way to hold your tubes to your design and have some method to check that they are all in the same plane. Do you have some kind of flat surface at work? For those without access to such goodness they find a piece of kitchen countertop or even a couple of sheets of MDF board put together. A large table saw top can do the job too. A common way to hold the tubes on top of a full scale drawing is with Paragon Machine Works aluminum tubing blocks. There are many different ways to successfully build a frame and we might be curious how you plan to proceed because not all of them work equally well. Some of your methods will be based on what tooling and equipment you have available.