In the Stern Papers at the Liddell Hart Centre for Military Archives at King's College London there is a 1/8 scale general arrangement drawing (which as I recall is very detailed) of a Mk I from the Metropolitan Carriage Wagon & Finance Co. Can't remember if it shows the sponsons, though.
The "Plans Pack" which I received from Bovington contains only a single side view drawing of a male sponson. There are, however, several detailed plans of the 6-pr gun, sight, mounting and shield. Let me know if these would be helpful to you (but maybe wait for a day or two - my e-mail server is "experiencing technical difficulties".)