The box is 430mm long,x 153mm high
It is 110mm wide at the top, and 90mm wide at the bottom. Side on ...the box tapers equally .
The box is made of about 3.5mm plywood.
It sits on some 20mm blocks.
I made the box by making the tapered sides first out of 3.5mm plywood and then making two rectangular pieces 146mm high by 84mm wide. These were glued centrally to the tapered pieces.
The top and bottom were made to the widths quoted but less 7mm for the lengths (ie 423mm long not 430mm). I also made another two rectangular pieces 416mm long by 84mm wide which were glued centrally onto the top and bottom pieces.
The top was then glued to the sides with the bottom loose fitted and it all clamped to keep it square while the glue set.
The bottom piece needed to be removable to allow access to the lights (if you look at pictures of the originals this is how they worked with two screws on each side) so two 84 mm wide blocks fitted at each end of the bottom piece and with it in place in the frame pilot holes were drilled for the screws then it was screwed into place.
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Sorry, I didn't take many photo's of roof box construction as we were racing to get everything completed for the 100 years of Morris National Rally.
For the sign two 2mm thick 422 x 146mm pieces of clear plexiglass were bought from acrylicsrus.co.uk (other providers are available) and we got a local sign writer to cut vinyls, a blue one with the police letters cut out which sat on top of a white one to mask the interior. (He also did the magnetic police lettering for the doors at the same time).
Once the plexiglass was finished and test fitted it was removed and the box was covered in a thin layer of filler and sanded back with 80 grit sand paper so remove the wood grain, then sprayed with filler primer, sanded back with 800 grit and then sprayed with the white paint (BMC Glacier white in our case).
For the electrics we fitted a 6V halogen bulb which was run off of 5 rechargeable AA batteries with an accessible switch on the underside.
I can't remember exactly what we did about a magnet to hold it in place, I think it was a large one centrally. It did the job for slow parades around the arena at shows anyway.
The feet and magnet had thin rubber strips glued to them to prevent scratching the paintwork.[frame]
That's how it looked when finished, hope this has been helpful
