I've decided to move an old floor air return to the baseboard. I am going to use two adjacent wall cavities and a ~30" grille.
Not sure what sort of duct I should use to run through the sill plate and mate to the grille, if any.
Is it okay to just cut holes in the sill plate and have no ducting at the grille, and then start ductwork below the subfloor? That will be easiest based on where studs and floor joists line up for this. A "standard" 10" wide stack header/boot won't fit without mods to go around the joists underneath.
Here's a picture of what I'm dealing with. 
Best Answer
Return ducts are commonly formed using cavities in framing and drywall. The vent opening is often nothing more than a sheet metal frame around which drywall is fit.
Use sheet metal, drywall, and existing framing to create your passage, and simply apply your vent grille where appropriate.