Soffit/HVAC/Bass trap/Light tray plan

I had some help from BPAPE a while back but I'm now getting to the point of doing the soffits. My soffits will be constructed completely within the GG/DD/clips enclosed space. Their purpose is to hold enclosed HVAC runs, a light tray, a ledge for rope light, hold all speaker/projector wiring and bass trapping with any open space after all that.

I made a 2D sketchup cross-section of how I plan to construct it. For the HVAC enclosures MDF was suggested but I'm considering switching to OSB sandwiched with gypsum board and a layer of green glue. This picture shows the minimal width section. Some parts of my soffit will be wider with constant width light tray.

I've color coded the picture:

Yellow - OSB
Green - gypsum board
Tan - lumber

The red L-brackets are Simpson Strongtie angle brackets. I used those in place of ripped 1x lumber to leave more space in the HVAC enclosure.

The ripped 2x4's will be screwed and glued to the ceiling. The OSB sheets adjoining the lumber will be screwed and glued to the lumber. Will this be strong enough?

I'm still a few weeks away from doing this, so I wanted to see what the experienced builders here think of my plan. Have at it.

Sketchup file is in the ZIP.

 

soffit cross section.zip 47.466796875k . file

I apologize for abandoning my own thread. I've had a change of plans. I had an HVAC guy out and he was concerned my HT room would put my already taxed HVAC unit over the edge. I think I'm going with a mini-split tucked into the soffit and a dead vent for fresh air exchange with the rest of the basement.

This means I can stick to a single 6" in and a 6" out. This saves me some soffit space for surrounds and bass trapping. In general, I'll use the same layout as above, but it should require less enclosed ducting.

Bryan, I forgot you spec'd MDF. I was a little concerned about the weight of the MDF, but my MDF boxes should be smaller and lighter now

Bryan, what do you think about lining the dead vent boxes with cotton duct liner instead of using flex duct?

Sounds good. I'll stick to insulated flex from start to finish.

Ted, that is essentially what I'm doing. Each flex will start in a larger box section filled with insulation and it will snake back and forth a few times. That part will transition to a narrower ~6ft long cavity of MDF/GG/Gypsum until it exits the room which will encase the insulated flex.

I am curious why the duct liner would pose issues. Isn't it usually faced so fibers aren't blown about? I figure I could build a larger box that is baffled allowing for more bends. Is it simply not as effective as the 6" flex running through boxed in insulation?

I've attached a quick drawing of my deadvent return line. The supply is similar but at the front of the room.

Thanks for the help.