Jen,
As Lee has said you can improvise - often graph paper is a good medium for laying bones on (carefully tape a few sheets together or get your hands on some of the lovely gridded permatrace for durability) it allows you to ensure that your 'blocks' are lying at the correct angle to your bone.
Health visitors have a sort of rolled up measuring board that they use for babies that I have always hankered after.....see http://www.quickmedical.com/seca/pediatrics/210.html - though they seem to only measure in 5mms.
Jacqui
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