Yes I know... Don't give me that measure twice business, it still happens, and when it does what are you going to do?
Having made this particular sill from scratch (and I mean from scratch, having felled and chainsaw milled the tree myself (more on the DIY chainsaw mill in the future)), I didn't really want to give up on it.
Lets add a bit of length and make an interesting and unique feature of it. Adding length to a board of wood is difficult - even if you find a grain and colour matching piece of wood, end grain gluing doesn't work. In short trying to make an un-noticeable addition is very difficult and will probably look scrappy anyway (and it's a bit boring).
Better we thought to make a feature out of it, make it look like a playful and deliberate addition - something cool to remember the comedy of a too short plank.
I'll go over the method I used.
Step 1: General Idea, and tools required
The more complicated and large you make your shape the harder it will be to get it fitting tightly, without a gappy joint. I found this one quite a challenge.
Tools used in this method:
Router (with flush trim cutter bit - the bearing on top type)
Jigsaw, coping saw, and/or bandsaw
Safety equipment (goggles, ear defenders, dust mask, dust sniper, etc)
Your choice of Wood glue
2 part epoxy and some saved wood dust (useful if you expect to have some small gaps in you joints - you should expect this!)