Ta-da! Bit of string with knots in it, threaded through the top vent and wrapped around the fan. It's at about a 20-degree angle, pointing downwards, but good enough - and invisible from the front, for those who care about form over function. A better solution would be some modelling wire, bent to hook into the top vent, come over the edge of the case, then hook around the bottom of the fan. But I don't have modelling wire; I do have string. Annoying that I have to do it, but now at least I feel comfortable migrating the thing to prod!
