air conditioner cover

Fun with molding! Build a 1/2" wide track that runs along the vertical window sills to the left and right of the A/C. You'll then be able to insert a removable 1/2" thick sheet of plywood that is cut to size.

Then, with a router, create a 1/2" groove in a board, cut it to size, and place it along the top of the A/C and plywood. (if you don't have a router, use molding to create a similar track)

...no drafts. Unlike the A/C covers from stores, it covers the accordian parts.

Installed and painted, it will run you about $60 (about $20 in lumber, about $40 for whynot). It will save you from ever having to remove your A/C. You can lay the thin plywood sheet and small board under your bed in the summer. Only damage to sills is some brad holes.

...if you have aluminum sills, you'll want to go with a pre-made aluminum track, and then screw it it to the vertical sill.

If you want to spend more than $60, use some plexiglass instead of plywood.

grrrrr.
a.
properly.
built.
sleeve.
would.
create.
a.
second.
vertical.
sill!!!!!!

It could supplement the metal piece that separates the two windows.
You'd have to brace it on both the inside and outside with something like padded tension bolts, so you did no harm to the sil (one side) and metal piece (other side).

the sleeve would be removable, and small (the height of your a/c, by about about 3". (it could be easily stored)

You'd have to have someone look at the set-up of the two windows, then figure out how to build a sleeve from wood (or some other material) given your unique set up.

Perhaps you could take a picture of the A/C to aid in the discussion.
...My idea might not work, in which case you would have to go with the ugly styrofoam board solution. Instead of covering the styrofoam with fabric, you could use pictures of alpacas.

If you used cork board (also a good insulator), you could have an instant dart board. ...good for hanging pictures of a nemesis, like that damn helpful whynot.

I too find myself to be lacking the DIY gene. Here are some basic instructions.

1) Go to Pinchick
2) Ask "the guy" for the AC insulation kit (it should contain foam insulation, plastic sheeting, and the quilted cover) and the no residue duct tape
3) Bring kit home and remove foam insulation (pictured above). Take a damp cloth and wipe down your AC, the window sill and ledge. Dry all surfaces befr you begin. Remove the tape and stick the foam around the edges of the AC including the accordian sides. Be aware that the sticky tape on this stuff works for about 12 days or until a good stiff winter wind blows through. You may want to reinforce with some of the duct tape.
4) Remove the plastic sheeting from the kit. Drape it around the actual unit and tape it in place using the afore mentioned duct tape. Pretend its just an odd sized Christmas gift that you are wrapping. Like all Christmas gifts, the goal is to make sure the sheeting covers everything and there are no places where the AC is exposed. If you have any sheeting left over, tape it over the accordian pieces and to the edge of the window to provide additional protection
5) Take the quilted AC cover out. It should come with elastic around the edges, but you may want to tape down to provide a better seal.
6) Go to IKEA. Purchase a nice set of heavy curtains (usually the have a faux velvet fabric they sell during the winter). Hang the curtains over your windows. You now have a really attractive covering to your ugly tape and quilted cover job.
7) Check occasionally during the winter. If you see the curtains moving you may need to retape.

Had a cover made for mom's EXTERIOR. It was made from metal, lined with Styrofoam and attached to her window frame with what looked like little clips. The interior did not have anything. Altogether it ran $150 but that was over five years ago. When we take it off in the summer, we leave it in the front area behind the garbage cans.