OK. Did not realize that the bulkheads went through the cabinet. That is a tough one.
But you do have an option.
Use the return bulk head for an emergency drain and bring the return through the cabinet and have it exit just above the rim of the tank. You can then turn the return down into the tank.
This would give you a main drain and an emergency.
The main drain (one you have now) would involve turning the 90 to face the bottom of tank. You would then adjust gate valve to get water level to just above the top of the 90. I like to drill a small hole in the top of the 90, just to bleed air after a power cycle or something. This small hole will not make noise as it only needs to be about 1/16" - 1/8".
You will then take the return bulkhead put a 90 on it and turn it straight up. This will need to be below the top of the tank, but above the waterline. Sometimes you may have to cut the 90 down some to bring it below the top of the tank, but that is easy. This will allow for an emergency drain if the main drain clogs.
Then your return is just a new hole in the cabinet.
I did this exact setup on my frag tub added to my nem and macro tank system. Worked well and was super quiet.
Hard to see it, but if you zoom in on pic, hopefully you see what I mean for the drains.
90 closest to front is pointed down and one next to it (behind it) is facing up.