I got one of the holders from Chronos for £26.95:

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It seems to fit well and is 'solid'. Even at that price it's going to cost a fair bit to get enough (though not 26!), and it's going to take a long time to make my own.

There's a version of it here with the proper square bolts:

http://www.rdgtools.co.uk/acatalog/Q..._Holders_.html

Slightly more expensive but I might get one and compare.