Bare with me as the early stages are going to look a bit "Blue Peter" ( British childrens TV show famous for making things).
The first thing was to mock it up in cardboard to check the proportions and check the relative angles against the photos i have.The other thing to note is that there had to be discrete modifications so it fits me.The actual base of the seat is a bit higher and the seat stop a little further back but oddly the finished article look better proportioned than the original in my opinion.
http://wwhttp://www.400greybike.com/for ... ew&id=1163

Before starting to build the actual plug it was important to make sure the bike ( on a bench ) was sitting level on its wheels and was absolutely vertical.
download/file.php?mode=view&id=1164.Back to basics with plumb line and spirit levels.download/file.php?mode=view&id=1165

Once everything was square an mdf sheet was fixed to the "subframe" area.This forms the seat base but also goes back further to provide something solid to mount the seat hump structure on to.
download/file.php?mode=view&id=1170http ... ew&id=1171
Vertical elements were added to build on, these set the angle for the front and back of the seat hump but are within the thinnest measurement width ways to allow for tapered sides.

Using measurements from photos and card templates as a guide all the flat surfaces could be added, effectively the shape of the seat unit with all the corners missing .These will be shaped using air-drying clay later.download/file.php?mode=view&id=1172

download/file.php?mode=view&id=1173

More mdf was used to blank the back of the gaps left for clay.Just to give support while applying it.Also more strips would be added to build the shape on the bottom edge where it flairs.download/file.php?mode=view&id=1174
download/file.php?mode=view&id=1175

Once the mdf was built up i used styrene sheet ( the yellow bits ) to bridge the gaps where the side panels were so a larger sheet could be stuck over it.The blue stuff is just high density polystyrene used to fill the bigger gaps between the tank and seat.The finished "tank" is longer than the original.
download/file.php?mode=view&id=1176http ... ew&id=1177
This will be shaped with clay too.

Thin (2mm) self adhesive neoprene was stuck to the front edge of the tank cover.This needs to be as close as possible to the thickness of the finished seat unit or it wont fit over the tank.The outside of this plug will be the inside of the mould.
download/file.php?mode=view&id=1178
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Started applying clay to the seat hump using the metal templates i made earlier (i told you it was like Blue Peter! ).This seemed relatively simple.I had a template for each angle needed but all with a matching radius.
download/file.php?mode=view&id=1180
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After leaving it overnight i realised the clay dried too quickly.It had shrunk away on the thin bits but was still workable where it was thicker.This was despite putting cling film over it to keep the moisture in.
download/file.php?mode=view&id=1182
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The obvious mistake was not sealing the mdf first.The clay was on a porous surface.Looking online i found out that you can seal it with pva wood glue watered down.I also found out that you can seal air-drying clay the same way.This would mean it should last until the fine glass matt was applied to stabilise it.I removed the clay ( can be used again ) and treated the mdf then re-applied the clay.
download/file.php?mode=view&id=1184
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