Yup, I used a template bit.
First of all I chisled away all the remaining wood, then put the template on and routed the sides. As the base of my template wasn't wide enough to reach the middle of the "chambers", I removed template, used the same thickness mdf to bridge the gap and removed the middle section with the same bit ( I don't normally use a template bit for hogging out but it needed to be replaced) and the router marks were cleaned up with sandpaper.
I have made an extra wide base for my router but in this case I couldn't use it as I don't have a 1/4 shank bit which would have reached the bottom. I can find 1/2 shanks 2" long but nothing in 1/4