Will screwing the power supply to the alloy be fine as I think I have some 5mm..
It will depend... IF you can get a good amount of the bottom of the power supply touching the heatsink, then the heat will diffuse into the aluminum and give a huge surfface area to bleed the heat off into the air.
Various power supplies I have seen and a few I own seem like they are designed so they can be heatsinked like that (in fact, I have a 12v DC power supply I just set on my shop floor and it works great), however, if their is even a *tiny* gap it won't help.. much. Easy to fix tho... I'd just fold aluminum foil exactly the size of the bottom of the power supply until it was just a tad thicker than the gap, and sandwhich it between the aluminum plate and power supply. This will absolutly help... and with your tent exhaust blowing on it, it should be sufficient.
However, if you still notice it running hot when summer hits, an engineered heatsink and an ultra thin layer of thermal glue is ideal because it is designed to provide 99.99% contact between the heat source & heatsink and transfers the heat more efficiently. Also, engineered heatsinks provide massive surface area to bleed off heat and take up less space.
I have never needed these on my power supplies, BUT I do use them on the motors of the "bucket coolers" I build to deal with overheating -- these cheap radiator fans work great for this, but they overheat because they need massive air flow... so I had to create the air manifold & attach heatsinks. These are only 5mm 5mm x 3mm... and 8 of these tiny heatsinks did the trick.