I am kind of proud of this one. I thought of it on my own. However I am also aware that there isn't really much "new under the Sun".
Some of the guys where I am currently doing some work frequently have to "pull square" or "cross measure for square" on some large metal frames. Here is what I came up with to help them out.

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ImageShack.usPurchased the magnetic squaring tool and the tape measure. Fabricated the aluminum bits from some bar stock and flat-bar. After assembling this is what you get.

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ImageShack.usNow one man should be able to stick the magnet in the corner, pull a diagonal measurement and repeat with the other corner allowing him to square up the assembly. This does not give a "true distance" measurement but it should give a consistent measurement between the corners which is all that is required to cross measure for square.