I'm looking to buy a square for my MFT as well, and did some investigating already. I started with Woodpecker and Starlett products in my mind, but came to the conclusion that there are other, more affordable, options available. (Albeit with less other uses, besides being square)
I'm talking about the 'engineer's or machinist' squares Peter already mentioned. In the UK they use the BS 939 standard, in Europe the DIN875 standard. For me it's important to use a square manufactured according to a given standard. I was hearing a lot of people complaining about cheaper squares that weren't actually square, or had some 'play' (over time).
The DIN875 standard stipulates 4 grades: 00, 0, 1 and 2 (in decreasing accuracy)
Tolerances per grade:
http://www.thome-precision.com/DIN875.html I'm thinking the least accurate grade 2 is actually sufficient (for my use). A din875 grade 2 square with a short leg of 300mm (~12") will have af maximum fault of 0,05mm, extrapolated to 1mm at 20m.
What I'm still debating (for use with the MFT) is to go for a flat square, or one with a back:

And what size I would get. I'm thinking about a 500x250mm model (20"x10"), as I'll still be able to use it in 600mm (24") high/wide cabinets.
What do you think?