What materials can be used to create cold air returns

I'm almost certain any occupied space is required to have a supply and return duct for HVAC.

Not quite sure if you can say it's a common plenum in the basement if the furnace is in another room.

Also - I've never seen a return in the floor. On the wall and in the ceiling. This is speaking from experience working as an HVAC controls field service engineer in my previous life. Running a return should be easy, you could get some flex ducting and run it above your t-bar ceiling.

Are you sure the air is coming from the return vent ductwork itself? It's not coming from the edges of the vent where the ductwork is attached to the vent?

I would hypothesize that on windy days, your attic is becoming pressurized by air entering via soffit vents or roof vents that are on the windy side of the house.

Air would need to escape to the lower pressure areas wherever possible. In theory, this would be vents on the downwind side of your house. But if there are air gaps where ductwork meets a vent or where a ceiling light fixture goes, those would also be ways to equalize the pressure.

If your attic were becoming pressurized like this, you'd probably notice a large gust of air coming from the attic if you opened the access door to your attic.