You can do an internal box, this is one I had on a bigger tank with the same kind of deal, bulkhead in the top right corner
I did mine "coast to coast" style but there's no reason you couldn't just make it into a small box around the bulkhead. Between this and controlling the flow I reduced the noise to a gurgle. I did NOT have an external stockman on the back so that might reduce it even further. It doesn't compare to a proper thought out overflow but it's a heck of a lot better than nothing.
I installed it with livestock in the tank. I drained enough water to give me access to the area, propped it up with a chunk of PVC pipe and used clamps to hold it to the sides. I left it like that for 24 hours so the silicone could cure properly and then filled it back up. If you have a spouse/kids it might be a good idea to send them away for the afternoon/evening while you're doing this... I invented a whole lot of new swear words!