Thanks to all the responses to my question. I believe I'll try a box beam even though it sounds like more work. Plywood comes in 4'X 8'and I need 16', so do I just offset where the sides join so I don't have a joint going all the way around at one spot?
Thanks, James

: You dont "need" a table saw (or even a circular saw) to
: "cut" sheet goods. Get a straight edge that you can use for a
: guide for your router and cut the sheet stuff with that. You can use one
: edge of another sheet and cut off a more portable straightedge guide from
: the first sheet and use that from then on. Use a small diameter bit when
: doing this to avoid excess waste. 1/4" should work fine. If you only
: have one sheet, then use a 2X4 as a temporary guide to get the guide strip
: and then use the straight edge of the guide strip to clean up the wavy
: edge that the 2X4 will leave.

: Once you have the edge guide, just router/cut off some more strips that
: you'll need to make your beam in the right size widths. I'd use thinner
: material than 3/4" and make a box beam rather than a laminated beam.
: The box beam will be much stronger and less flexible.