Im reading my post again and im not sure if i was clear enought with what i said about removing turbulences inside the PSU shell
@amirzaim is tricky to explain with words, but i made a drawing that shows the "problem" a lot better
The PS3 uses "axial" fans (also named "centrifugal fans")... this means the air inside the fan shell is forced to spin "against" the walls... in other words it tryes to escape in radial directions
And the fan is spinning in counter-clockwise direction.... so inmediatly after going outside of the fan shell (into the PSU shell) it have a huge tendency to go to the right direction
Thats a real problem in the factory design because the PSU shell doesnt have "air outakes" all along his back... and it can be fixed because there is a circuit board inside the PSU

But in your case you are going to remove the circuit board inside the PSU... so... this allows you to redesign the PSU shell to improve his geometry
Basically... the idea is to create 2 "walls" (represented with the blue lines) to remove the 2 turbulences at the inner corners of the PSU shell (represented with the spiral red arrows)
If i where you, im not sure how i would do, you could "cut" the PSU at the corners and seal them with glue and plastic pieces. This is going to look a bit ugly but seems to be doable
The other option i see is to buy a (very) big tube of silicone and fill the corners with it, the only payback is that is going to increase the weight of the PS3 a lot because is needed a lot of silicone to fill that corners... but well... it should work
Maybe the best solution is a combination of both... just crop some plastic and try to seal it very well with silicone
And remember, you can use a plastic globe with a finger "wet" in oil to apply the silicone with your own hands... not only in the 2 corners in my drawing but in all the edges (all the angles with 90Âş to soften them)
Im not sure if this mod changing the geometry of the PSU shell is going to improve the CELL/RSX temperatures... or the cooling of the PICO PSU circuit board... but anyway... is the kind of project where the quality of the "final product" is doubtful (because it depends of how you do it), but the principles of why you are doing it are right (if my analisys of the problem is correct, and i think it is)