I was original going to fit this type of bracket
Ideally I like the ones that are fitted to bookcases. ie 4 clips, 2 on each side (similar to kitchen shelves).
But other than fixing a board either side, I can't figure out how best to do this.
If I do fit a board either side, it will certainly make it look better inside the cupboard. But I've never fitted bookcase version that can be seen here