SSDs are fairly low power and low heat, so they don't need much in the way of airflow.
my case has the PSU at the very bottom of the case, and the mobo & slots above it, and as long as you're able to leave 1" or so of air space between the SSD and PSU, the PSU should be A-OK. would that install even block the PSU's air intake? on mine, the PSU's air intake is at the bottom, so placing something on top would have zero impact.
but ya, once you land on eBay, you can find plain 2.5/3.5 brackets and PCI brackets - look at them both and go from there. the 2.5/3.5 can be had for less than 3 quid (from china, of course), and the PCI might run you a tenner.