Edwin Pawlowski wrote:

Put the vents in the ceiling. Forget the air handler.

Easier said than done. That is one reason the OP asked the question of HOW?

1. Buy some 12"x4" floor registers with dampers from Home Depot, about $10
for 2 brass-plasted versions for low-traffic areas or $10 for each for
solid brass in high-traffic. OR, buy some $20 13.5"x13.5" Hart and Cooley
registers. Both types fit nicely between joists on 16" centers.

2. Put a supply vent in each room, away from outside walls, ideally near
the ceiling, eg a screen above each bedroom door off a central stairwell,
and install return vents near outside walls by drilling a hole in the
ceiling (taking care to avoid wires and pipes and joists), then cutting
an outline for the register from the floor above with a Sawzall or a sabre
saw with a long blade. Install grates with drywall screws into studs from
above and below.

I did this in a basement ceiling to help warm air move up into a house.
The lower grates weren't required for cosmetics.

Nick