Better method for installing in-window air conditioner?
Take two pieces of 1/2" plywood, that have been cut to fit the width of the window and are 1-1/2"-2" taller than the A/C unit.  

Cut out the outline of the A/C unit from the center of the plywood pcs. so that you've got a "collar" that can be slid down, like a yoke, or otherwise fit around the A/C unit.

Cut out "windows", in both pieces of plywood, on either side of the area where the A/C unit will be; maintaining at least 1" between the window jamb and the area the A/C unit will be and these "window" cutouts (so the plywood won't break).  At this point, both pieces of plywood should be identical where the cutouts were made.

Cut two pieces of Lexan (or similar) that is 3/4"+ larger than the "window" cutouts in the plywood (another reason to leave a 1" space between areas cut-out).  Glue (with something that isn't too permanent) into place and sandwich between the two pieces of plywood using carriage bolts to secure the plywood together.  Or, glue, if you're confident all is lining up perfectly and will fit properly.

Using (black) foam pipe insulation, wrap the plywood edges and staple into place (staples on the side that will be outside...to prevent water from getting inside the tubing); including the edges that will abut the A/C unit. And give it a trial fitting around the A/C unit that is already in the window, to be sure the pipe insulation compresses enough for a proper fit.  Shave the plywood where it's too tight a fit.

Note: a thin (half-width) strip of duct tape along the seam of the pipe insulation and plywood cutout will create a sort of "drip edge" or seal to divert rain water better than the staples alone.

Note: if the A/C unit tapers or has indentations at the bottom; such that the plywood would leave gaps if slide down like a yoke, first try cutting the shape of the A/C unit so it will slide into the plywood frame.  If this won't work fitted pieces of insulation board (or piggy-backed pipe insulation), duct taped into position, will do (these will be what gets replaced each year).