Sure can do, John. The whole process only took a couple liesurely hours (7-beer job).
Here'* some better sort-of daylight pics:
All you need is some adhesive-backed strip magnets (flexible) from a craft or hardware store. I used 1/2" wide, and also a sheet of material. You don't have to use the magnetic flexible vinyl like I did. But it does come in white.
Go to www.mcmaster.com
and search for magnetic sheets or flexible magnets.
You could even use simple cutouts of polycarbonate or acrylic, shape them the same with the magnets, scuff them, and paint them. They'd be thicker though. I have noticed that mine sag just a little across the vent, so you probably don't want to go thinner than the magnetic door sign material I used unless you don't care about looks. You can see in the closeups that I had to double-stack the strips (carefully orienting them so they'd naturally stick together magnetically as well), overlapping the corners opposite each other. That gave me enough height to clear the vent. One strip thickness was just a LITTLE short. If you buy some thicker stuff, you can do it in one shot.