Thank you!
It was known to me as closed loop, but search for "return manifold" gives additional information. I did a fast search (in depth will do a little later), result shows:
1. Closed loop inside the tank, at the top.

a) The "T" are opened (without nozzles or other snail guard). Can you give fast note from memory about not bulky solutions for this?

b) The loop is zip-tied to the holes in the plastic rim of the tank. I had long spray bar at the top in this manner, it interferes with cleaning glass. Is there other way to fasten the loop on the tank, to keep it maybe on the rim, so access to the glass will be not obstructed?

c) Way to eliminate sound of splash, if the loop will be mounted above the rim: Lockline with nozzles (can you point me to the info on connecting LockLine details to the PVC construction? I ordered all details, what were available in local online supply, and way to connect them is clearly missing.), or PVC 2x 45 degree elbows (again, have to be protected from snails). Any other options. that I missed?

d) this is a single loop. Now I have 4 sources of the flow, if one fails - others will work. Is it possible (and how) to make a double manifold, each fed from independent pump?

2. Matt's cube:
Way to mount above the tank, on not removable canopy. My tanks do not have canopies at all, and building one will bring a lot of inconvenience in my maintenance - restricted access and not removable object. Any similar idea, but easily removable?

Penductors are opposite to my task: even Maxi-Jet 1200 is too much. Seios, Koralia and reflected flow are more suitable, soft and diffused flow, but a lot of it. With possibility to create easily re-definable zones with different amount of flow.

Areas of high velocity jets are a dead, lost space in my tanks - nobody is able to tolerate that, speaking from my humble practice, even a very high flow corals, like scleronephthya and dendronephthya.

More ideas are very welcome!