Hi Niall
I posted some plans for cajones on my scribd account:
http://www.scribd.com/doc/18545454/Cajon-PlansThe quinto and tumbadora cajon measurements were taken from Yoruba Andabo's cajones c. 1994. I think they sound great but the trend seems to have been toward larger cajones lately. The caja dimensions were taken from Amelia Pedroso's group about the same time.
Also check Thomas Altmann's site for his step-by-step account of the set he made.
saludos
Barry