Heavy paper seems to be the best template material, I used traditional brown paper that tailors and the like used before plastic. Otherwise, wall-lining paper, the kind you put under wall-paper to cover cracks and blemishes, would be an alternative. Make up a large sheet, I used parcel tape to join two (1 yard wide) pieces and to add a couple of 'wings' to hang down over the doors. Bryan recommends the use of double sided tape to stick the whole thing down onto the body-work. Cut out slots for the seat-backs and steering wheel and re-tighten. (I suppose it would be best to set it all up with the chairs and steering wheel removed, but I didn't. If you do then the chair and wheel slots won't cause problems when tightening and can be done later ). Bryan will need to know the height of the seats and wheel above the plane of the tonneau cover. You'll need a bit of clearance for the centre zip. Add markers for the split if you want to be able to fold the edge in before rolling the drivers section back. Pictures and rambling commentary on you-tube http://youtu.be/z3FgwQ7PCTA so that I could show Bryan what it looked like.
Once you have it all tightened, the rear corners will need some kind of fold-over, Simply pleat the paper so that it forms a corner, and mark the paper to show the creases.
The depth of the overhang needs to be at least 2 inches all round, and I would recommend a degree of generosity here, it's easier to cut bits off than add later.
Using the brass rubbing technique, mark the edge of the car body onto the paper and follow it round with a line at least 2" wider to show where the edge of the cloth should be. Most Midge bodies are shorter than mine, but I think the rear portion should still have a horizontal edge.
I put my own pop-studs on afterwards, bear in mind the size of the attachments you intend to use. 'Lift the dot' and Turnbuckles need a bit more space. There will be a hem with a covering strip, the fixing should go through the folded over hem but not the covering strip.
My first Tonneau cover is in black acrylic which has the advantage of "black goes with everything" and is light-weight for storage and postage. There are other colours, cloths and weights available. This model has racing screens rather than a windscreen, remember that you have to get your hand in to undo the fixings, a Turn-buckle may help in cramped areas.
Different areas of the cover have different functions. The driver's flap won't need to deal with high winds the way that the attachment of the front of the zip on the passenger side will, and the flap hanging over the passenger door may have water coming up from the front wheel.
When I have Bryan's approval, proof-reading etc. I'll add any bits he deems suitable, so don't take this as gospel until I've removed this sentence.