Hay Xion X2 I feel that if you are pulling cool air from the bottom of the case for the bottom rad, then you should also be pulling cool air up from the bottom for the PSU and exhausting out the front, as hot air rises it want be sucked back in from the bottom. You would just need to inclose the PSU in the custom drive cage with only a hole to allow your cables through. That way you could put a chrome ring around the hole like the ones you are going to use for the tubing.

As for the challenge with the custom drive bay. "By the way I should let you know that I know nothing about this case" But I have work with steel fabrication for 20 years.
Do you really need to keep the section that crosses the HHD bay?
Would three locking latches be enough to hold the door on tight?
If so you could remove this section along with the mounting bracket that the standoffs use. Hope you understand this next bit.

If the custom drive cage is say 150mm wide, then cut the support bar down by only125mm. As the support bar is a channel you will need to cut a piece out of the top and bottom edges, just mark where you want the bend to be, e.g. at the 150mm spot that lines up with your custom drive cage.
You will need to give yourself say 1mm more than the 150mm mark for the metal to stretch and give you room to put the custom drive cage in.
After you have cut the edges out and before you bend the support bar you should drill a hole in the support bar, (actually you should drill the hole before you cut anything, this will give the support bar strength and stop it from moving around).
One all cut and before you bend round off edges then bend to 90 degrees with pliers.
Now you can mount the support bar to your custom drive cage and have room to boot.

Hope this all makes sense Xion X2, and keep up the good work.