Concrete nest boxes

Thanks for the pic Paul - sounds like this would be an interesting guy to speak to, if he has also been tweaking his nest boxes. This looks a little smaller inside than a standard sideview? That is something else I have been thinking about - perhaps a little smaller and cosier could be attractive to them? I know the nest box at my old lady neighbours is on the small side, with a low ceiling. I imagine they might like "just big enough" but no bigger. All of this is assuming there is something better on offer of course, because they will nest in most boxes if they have to.
I think I can make the concrete mold out of scrap wood, effectively for free, since the design is simple enough. The rubber mold is probably way overkill now that I have thought about it a little more.. Any idea how thick the holes are in the concrete boxes? (I mean how deep - i.e. how long is the tunnel that the bird goes through?) Longer entrance holes will help against predators quite a bit I should think, so perhaps they prefer them? The sparrows and starlings love the holes in my house wall and those are a full brick thick (70mm ish?).