The air handler is located in the garage mounted on a box next to the water heater. The return air vent is located low on the wall in the family room which is under the staircase. So there is a void as large as the support structure for the stairs and the high of the support box with little or no insulation. Question, can I fabricate an insulated enclosure (tunnel) just a little larger the intake grille to increase the overall efficiency? Your assistance is appreciated.

Adding insulation cannot hurt unless you restrict the opening or inside of the return shrinking it down in size.

So if the tunnel is as large as the air handler intake filter and the intake grille it would be and improvement?

Insulation is always an improvement BUT you need to be sure that no other ducts or areas of the wall are being used for return air purposes. I am not there to look at it and am just going by your description. Most systems are installed today with to small of a return air to start with so that is why I mentioned not making it any smaller. Also the garage area has to kept completely separate from the living areas of the house for fire code purposes so do not go cutting any extra holes etc that would allow a nonsealed area to exist between garage and house.

The metal duct and the penetration between the garage and house must be kept sealed all around. It is all a part of the fires stoping section of the code.

Thank You