The shelf layout is progressing well. The current job is to fill in the gaps around the corners with foam blocks. I have tried using a table bread knife and a sharp hobby knife. Both of these are hard to push through the material. Cutting large pieces off was easiest done with a jig-saw.
Shaping foam with a Multi Tool
After the glue had set it was time to shape the foam roughly to shape. The objective is to remove the sharp corners of the foam and try to get a more natural shape without using too much plaster in the next step. I had a go at using a knife but it's hard work and you get a jagged edge. I found using a multi tool much better. The oscillating blade is easy to push through the foam and you get a nice smooth edge. The only drawback is that you end up with lots of sticky foam slivers.
The next step is to cover the foam witheither goop or plaster. I am leaning towards plaster as it's quick and easy to mix and there's no alcoholic smell (the goop has alcohol to prevent mould growing).