Daniel - I use a square I bought from Enco (638-7628), but the Enco web site says that item has been discontinued.
http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INLMK3
You could call Enco and see if they have a new number for that type of square.
TS Aligner, Jr sells the same square that Enco used to sell, but for a much higher price tag. Look here and scroll down to the squares. The 8X18 square is $157.62 - Enco used to sell it for $53.78. I know it's the same manufacturer because I have a smaller square from TS Aligner and the big one fro Enco - identical.
You can make a large, thick, accurate square from a piece of MDF using the 3-4-5 method. You can make it as thick as needed, and it would be fine for shop use. Might be a bit awkward hauling it to a job site though.
Regarding the question you asked Rob about the pin - yes there is a slight amount of horizontal movement of the guide rail, even though the pin is bottomed out in the rail slot. I simply push the saw a bit to the right so the play of the rail on the pin is eliminated. Even forgetting to do that, my cuts are as square as I need them to be. Our medium here is wood, after all.
If you have not already done so, please read the related threads at the Festool Owners Group, where this topic has been discussed in great detail. (Links to other forums are not permitted here at SMC)