The Bow bumper is one of those fluid jobs to do.  You don't want to waste time and make it too soon or leave it too late and hold up completion.  This afternoon seemed like an ideal time to tackle this task.  I hadn't put much thought into it. Only "several" hours whilst trying to switch off my brain prior to getting some sleep!  If you've built an IOM you'll probably know exactly what I'm talking about. 
So I knew it was to be 12mm thick (min class rule 10mm) and a quick measure made it 17mm wide.  I had an off cut of Cedar which would make an ideal plug and was near enough the right thickness so happy days.  Cedar is a great wood for this type of job as it has a pretty true straight grain, isn't too dense so makes shaping easier than hardwoods and is naturally oily.

After a little planing and a fair amount of sanding I had a Bow bumper plug.  Once I was happy with the shape I coated the plug with Carnuba wax rubbing it well into the grain.  This saved the time of varnishing etc.
Then I went about making a little frame to form the sides of the mould. I drilled some holes in the sides to release the overspill filler as you have to take into account the volume of the plug whilst pushing it down into the mould.  I had thought about making the mould from CSM GRP but WOW that didn't appeal to me due to the time and effort it would take.  So I decided to experiment with a 2 part wood filler.  After mixing the filler I spread it into the mould channel.  Sprayed the waxed plug with silicone and pushed it into the filler.  I was on the border of not mixing enough filler for the job but a bit of scrunched up news paper helped considerably to pad it out and also helped retain the filler in the mould channel.

An hour later the filler was dry enough to remove the plug using screws to help leaver it out and after a quick file the top was made flat.  Overall I'm very impressed with the finish of the mould ;-) and the relatively short time it took to complete ;-)