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Not too many illustrations come to mind; there are a few clinical (profile, plan and detail) drawings to be found in Steve O'Shea's Colossal squid fact sheet, and at TOLWEB's Mesonychoteuthis page.
They're not illustrations, but pages 7 and 8 of the Tentarcticles (Mesonychoteuthis / Colossal Squid) thread in the Physiology & Biology forum feature photos of the lifesize Mesonychoteuthis model produced for a Discovery channel special. Quite startling, especially the photos taken in a dark swimming pool.