I know what you're talking about. We had the same problem.
We finally found the Frigidaire FRA064VU1 Low Profile which fits into windows up to 41 1/2 inches wide. We still have to seal around the edges, but it looks a lot better than what we used to have to do.
Amazon has them for around $300. You'll find it on the Frigidaire web site about halfway down the page.

Here is a link that might be useful: Frigidaire A/C

Once you have a window AC you can't worry about "looking nice" ;) I'd get some plywood and cut it to fit the openings. Paint it out and you are done. It is best to try to secure it somehow for security reasons.

concerned about noise of running AC
Thoughts on which would cool better.

LIve in SouthEast Pa - West Chester
High humidity and high temps in summer.
Area for AC is a long rectangle - 3 rooms long. total 585 sq ft. 15 feet wide X 13 long lvg, dng, kitchen.

Choice 1. 15,000BTU -
window AC would get West (hot afternoon sun)
this is the middle (dining)room AC air would flow mainly across room to the opposite wall and have to branch right and left to flow to the other rooms.
OR
Choice2: 6,000 BTU
AC in south window which is under porch roof -
this window (living room) would flow air straight (length of rectangle) to all the rooms?

Carol0512:

Without knowing all the details of your home's construction I still feel that neither of your two options are going to be satisfactory.

Doubtful that Choice # 1 will give reasonable cooling to the other two rooms without some type of fans located to move air into those other rooms. Will probably overcool the room it's located in along with drafty feeling from high air flow...and don't forget the noise factor.

Choice # 2 even though it's located under a porch roof is way undersized to cool 585 square feet. That size unit is probably better suited to cool 200-250 square feet at most.

Would seem that you would need at least two window units if that's the way you want to go.

A better solution and much more expensive might be to look at a mini split AC system with at least a couple of air handler units within the space mentioned.

details of twinhouse: attached with side yard.
built 1886, three- storied brick with attic and basement.
1st flr 4 rooms, 2nd floor 3 rooms, 3rd floor 2 rooms access to attic no insulation in attic (suggestions welcomed).
Plaster walls, storms with screens.
Front of house faces South with 8-10 ft porch roof
side window is West
back is North. exposure.
. Attached neighbor does not heat his house, kerosene burner only.

Good suggestion using two AC. The FRA 6,000btu is the right size for my window opening which is 27 inches and the unit is 23 inches. Walmart has the units on sale for $280 and my electric co offers a rebate of $25 per unit.

fill the side gaps with painted wood?
some type of foam?
need suggestion for shading West window AC -narrow cement path next to this window.

foam sheathing board like dow blue board.
take off plastic sheet coating & paint.
easy to work with, waterproof, air tight (when sealed at
edges) inexpensive, adds insulation value.

best of luck.

A 6,000-Btu/hr ought to cool that room area if you use a wind machine 3300 to move air around & back to A/C; do that & it will cool even; read my page on my system.

Here is a link that might be useful: Optinizing Windoe A/C Performance