Having followed my own advice in a comment on Aloysius' answer, I did a DuckDuckGo search for "home made track saw" (or something similar) and came up with a 90-second video (on the tube of you) from Tom Silva of This Old House fame showing how to make a very simple track saw from scrap plywood. (This was only one of hundreds of videos & written instructions I found, choose your favorite.)
Not having a lot of scrap plywood laying around, I bought a sheet of 3/8" ply, carefully ripped 2 strips about "yay" wide each off the long edge (super wife is an excellent assistant in-feed table/material guide) on my father-in-law's table saw, then took the pieces home.
I glued the two 4" strips to the plywood and "clamped" it with a pile of lumber over night. In the morning, I ran my circular saw along each edge of each glued strip, once with the wide side of the sole plate next to the attached strip, once with the narrow side.
This gave me 2 8' long track saws. I cut one into a 5' and 3' section.
Despite these being made of some rather wavy plywood (can you get 3/8" ply that isn't warped in some fashion?), they worked extremely well for all the rip- and cross-cuts I needed in sheet goods for the shed I just built.
- at only 3/8" thick, the act of clamping each end down on my cut marks bent the track straight to meet the flat sheet I was cutting
- Using the track saw makes for effortless, nearly brain-dead straight cuts at any angle across the sheet without having to focus on a chalk/pencil line
- This was exceedingly handy since my saw seems designed to throw as much waste as possible toward the user's face when focusing on that cut line.
- I can stretch to the other side of a 4' sheet and still have a straight line
- I don't have to draw/snap a line all the way across the sheet, 2 simple marks is all that's necessary.
This is a very fast to build, incredibly simple and easy to use jig that I wish I'd had 20 years ago!