There are many different ways to build a soffit but this is definitely my favorite way.
How to build a box around heater in attic.
Make the box of 2 by 2 inch lumber with two side ladders with a top and bottom plate and vertical rails in between.
I leave in nh so in the winter time it gets pretty cold up here.
Attach the two layers of foam board together and apply them to the hatch.
Position the box over the opening making sure it is square.
Elevate an enclosure in a basement or garage.
Slit 3 inches in on the cord s outer insulation from the cut end using a utility knife.
Pull the insulation back from the two wires inside the cord.
Cut a piece of foam the size of the exterior dimensions of the box for the lid.
Cut off the end of the power cord using the blade of a utility knife.
Clean the floor around the attic stairs so the tape will adhere well.
Use concrete blocks or a concrete pad to set the base of the furnace at least 4 inches above the floor to prevent any water from reaching it.
Cut out two pieces of rigid foam board that are 1 4 inch smaller than the hatch so homeowners can slide the attic access panel open easily.
Build a framing box the length width and depth of the duct you will cover.
This video shows how to build a soffit around ductwork.
Doing these two things would dramatically reduce the amount of warm air getting into the.
Researching the web i found some opinions that folks say consider building a large insulated box around the entire furnace leaving an opening at the bottom for combustion air intake.
Unfortunately my house furnace is located in the attic.