In 30+ years of searching I have never found a drawing, plan, sketch, model or photo of FV426. Others have also tried particularly Chris Gibson with whom I discussed how the vehicle might look knowing what a FV421 looked like & the OW launch requirements. He did a very sympathetic set of drawings of how it was likely to have been, it was for an inclusion in a book, but the publishers dropped the chapter because they wanted to focus more on aircraft engines.
The best collection of OW stuff is at the PRO filed under WO 291/2214 this includes memos from the Army Operational Research Group & other correspondence. They have FVRDE Spec 9243 that covers the FV426, but no drawings.
As you may have noticed my avatar is OW (& not Malkara as commonly supposed)