I've been working on a case too, though mine is fully enclosed and will use a fan to draw air out, which in theory should be better in terms of dust accumulation than pushing air in. Though I guess if you really want you COULD turn the fan around and push air in instead. I'm still very early on in the design phase and although I have gotten the mounting, internal clearances and almost all of the port layout sorted (mic & speaker port still to be done) I've got a few things like fan, vents and button extension to sort out yet.
I'm using the design of this case as a way to learn how to use cura & sketchup, so it may take a little while to have something to show for it, but my intention is to make a design that I would be happy to pay a modest sum for the printed version of.
To the OP, thanks for the idea r.e. fans