After yesterday's efforts had dried, there were a couple of problems. Firstly, the corners had dried quite hard, and also some ridges had appeared where the material had expanded a bit:

So, I decided to have another attempt at it - this time, rounding edges.

The best thing to cut foam is a band saw - I don't have one of those, and nor do I have the space for one (realistically). Yesterday, I'd been using a hacksaw blade, but I remembered I had an electric carving knife. After a few practices, I cut a new base section out.

This is much neater than the hacksaw blade! I also cut it 2cm smaller than the other one, because I would be using some thin strips to round the edges more than cutting it.

I then put a 20mm chamfered edge on it:

And then glued the thin strips to the sides and front (there isn't going to be one at the back):

I then used a lot of masking tape to hold the thin strips to the chamfered edges and top:

And some weights:

Tomorrow, I'll release it and have a look. If I'm not happy with the results, I'll start looking for some trimmers...