Concrete nest boxes

I'm going to repurpose one of my existing wooden boxes as a robin box (cut the front in half), and then half 3x concrete: 28mm x 3 (this one), 32mm x 3 (if I can find 32mm pipe for the holes - for great tits) and then a starling box. That will be all, so less in total but hopefully more attractive boxes and targeting four different species! I've kind of given up on sparrows tbh, they haven't even looked inside any of my 32mm boxes and they are everywhere in the garden.. maybe they'd like a concrete one too?