It might not be illegal in your area, but it would certainly be very irresponsible if you don't have at least one fire extinguisher in your home. Yes, I know they are ugly dust catchers. However, it's easy to make good looking covers for them.
Make a sconce or wall light mock-up out of paper or cardboard, then use that as a pattern to cut up some old Plexiglass. Remember, sconces have no back. Join the pieces with shiny bath-tub silicone.
Once the silicone is dry, cover the outside corners with duck tape and trim it with a ruler and a sharp knife to look like old-fashioned leading.
Get some "Liquid Stained Glass" from a craft store or yard sale and paint the outside with it. You don't need a lot of colors. Amber Frost works fine, though you definitely can get artistic and paint a family crest.
Glue a strip of leather or cloth to the inside top, so that a finger wide strip sticks out the back. When that glue is dry, line the cover up so that it neatly covers the fire extinguisher, and nail or screw the leather flap to the wall. It will be the hinge. The weight of the cover will keep it snugly closed, yet it is easy to lift to get at the extinguisher.
If you have power nearby, a low wattage night light in the cover adds a really nice touch and makes it easier to find the extinguisher in the dark.
Do you have a picture you could post for us to see? This sounds like an easy craft to do.
Note: When I paint on plexiglass and don't have the "glass paints" I spray the plexiglass first with a matte finish and then I can use any paints I already have in the house on it.