Like many Instructables on the site already, this is a recipe I adapted from 'Winter HVAC Hack by iectyx3c' on instructables to block cold air drafts from my Central AC ceiling vents in the offseason in my single story 1950's ranch home that were condensing and dripping water.
My plastic vent covers have a center hole in them where a screwdriver can be inserted to twist shut the inside duct dampers (which are insufficient). If your application does not have this center hole, you will need to adapt this method with another way to attach the cover to the vent - perhaps a hook or by looping the shock cord over a louver.
These covers are hand made from 3/4" styrofoam & 3/4" rubber foam weatherseal and are secured using clear cord locks & 1/8" shock cord with a small binder clip handle. I needed enough for fifteen 10.5" x 10.5" covers and I had more than enough left over for at least 5+ more. Adjust the recipe per your dimension requirements.
Perma "R" panel foam 3/4" x 13 5/8" x 48" @ Lowes (part # 15350) $8.06 (6 x 48" sheets in this package)
Frost King Rubber Foam Weatherseal 3/4" wide x 5/16" thick x 10' long @ Lowes (part# 96112) $3.48/roll (I used about 6 rolls)
1/8" white shock cord @ Amazon.com (Paracord Planet) 25' for $7.49 shipped
7.5mm clear white barrel cord locks @ Amazon.com (Uxcell) qty 20 for $6.63 shipped
1 box small binder clips 3/8" (9.5mm) capacity $2.93 shipped
lanyard or other small hole punch
Step 1: Cut Foam Covers
Using a sharp knife and your ruler, cut out the cover dimensions & quantity that you require.
Tip: Use a sharp knife and 'pull' it through the foam rather than try to 'saw' it or push down to 'cut' it like a sandwich. I got a much cleaner line by 'slicing' it this way.
Step 2: Cut Weatherseal for Covers, Create Hanging Cords and Insert Them Into the Covers
Unroll weatherstripping over each side of your cover and cut to length (leave an extra 1/4" as the weatherseal will retract a bit) on each of the 4 sides. You want a tight seal in the corners between strips of weatherseal so again cut them a little longer than needed to ensure a tight fit. Press down to secure the stripping to the foam. Once all covers have weatherstripping on all four sides, create your hanging cords.
Cut roughly 6" lengths of shock cord - one per cover.
Squeeze the binder clip handles together and remove each one (2 total) from each black clip portion - you'll need one silver handle for each cover.
Install the hanging cords into the covers:
Tie the other end of the shock cord onto one of the silver clip handles.
Use your lanyard or hole punch tool to poke a hole in the middle of each cover.
Thread the shock cord through the cover with the metal clip on the 'vent side' of the cover.
Grab a barrel cord lock and put it on the end of the shock cord that you threaded through the hole, on the 'room' side of the cover.
Step 3: Install Your Covers!
Squeeze the small clip ends together and insert them into the center hole in your plastic vents. On mine this is a screwdriver hole to close the inside vent blockers (that don't work well obviously). Once the clip is installed, squeeze the barrel cord lock to adjust it up and pull down on the shock cord lightly to create upward tension between the cover and the vent.
Once the cover is flush on the vent, take the excess hanging cord and wrap it around the cord lock to keep it up out of the way.
That's it! You're done!