Cheers Grahameo,,, I'll be having ago at making one this weekend - pics too follow. I bought the sheet of 3mm Aluminium online for about £40. It came wrapped in 2 sheets of thin MDF which is perfect for making a template! Bonus! I'm guessing my biggest problem will be grinding off the various fixings from the old undertray that are rusted firmly onto the old gal?
Regarding underbody rust... I've just bought a car that looked fantastically clean underneath - all undersealed. It had been sitting for over 12 months and not driven regularly prior to that. When it went for its MOT they found various patches of rust under 'blown' underseal. (Now all sorted - thanks to Cory in Rugby). It is recommended that underseal is moistened occasionaly - either with water or with an annual oil spray. My car rotted from the outside in. I reckon some of it was where previous MOT inspectors had scraped at the underside with a screwdriver - looking for rust caused the rust! Maybe this should be considered before making a full body undertray?