I'm not very good at describing things Looby, but if these inserts fitted your nest boxes okay, what about cutting a bit of wood, only needs to be about 2inches thick and putting it underneath the back of the insert, just to get your slope? then, if the nest boxes were in a line, laying or fixing a plank of wood along the bit at the front that collects the eggs to stop them pooping in it?
If it was hinged, you just lift it up when you get home and hopefully all your eggs are inside the collection bit? As long as they are not all in the one box!!!
![biggrin :D](https://chat.allotment-garden.org/Smileys/green/biggrin.gif)