The illustration below is what I use at our 4 plex. It consists of a 2×4 that fits into the window sill with a little wiggle room (1/8 to 1/4″ ) to both sides of the window jambs.
I fasten this to the sill with two or three 3 ” deck screws with the front edge of the 2×4 tight against the window stool. The part you set your plants on! I then install a 1x 6 on top of it, (see below for other options) usually flush with the outside, rear, edge of the 2×4 with 1 5/8″ deck or dry wall screws , I then install a cleat on top of the 1x 6, using 1 1/4″ deck or drywall screws, a 1x 2 will work, with the back edge, of the cleat about 1/8 to 3/16 in front of where the front /inside edge of the lower sash meets the 1×6. This assures a bit of back pitch to drain any water in the AC case to the out side. This cleat does not have to be removed when you take the AC out of the window. The AC itself has a frame around it, the part that holds the expandable side curtains, This frame is what hooks onto the cleat from the back side. This is the part that normally hooks against the window stool. The top of this same side curtain frame hooks on to the lowered down sash as normal. If you cut and fit some tight fitting pieces of the 1×2 into the lower sash track, from the top of the lower sash, on both sides, to the top of the window frame the sash cannot move up and the AC is wedged in place. You could also make and screw smaller wooden stops in to the track on side jambs just above the lower sash to keep the sash from moving up. Some AC’s come with metal angle brackets that can be used for this as well. I like to use the full height tight fitting boards, It’s just a few less holes in the window frame.
If someone tries to push in the AC the cleat stops the bottom from moving in, and they cannot lift it up, since the sash is wedged down on top. If it tilts forward, the sash cannot go up, and the AC just wedges against the upper sash.
The whole assembly also serves to lift the AC above the bottom rail of the storm window and it lifts the AC unit above the window stool.
Usually a 1×6 is thick enough to lift the AC above the Storm Window. If not, I modify the design to get more height by using a piece of 5/4 (5 quarter) pressure treated decking which is a full inch thick, 1/4″ more than the 1×6, and the same 5 1/2″ wide. This decking also comes in a thicker version I e it is an 1 1/8″ thick which would lift the AC by a total of an extra 3/8 of an inch. Menards and HD have both sizes. Menards stocks shorter pieces, I think 6′ as standard.
Another modification, to make it even more secure, would be to add a cleat to the AC case itself. I have not done this my self yet, so the following is what I hope would be a good idea! Standard window sash are 1 3/8″ thick so if you were to secure another 1x 2 cleat to the top of the AC, at least 1 3/4 ” behind the side curtain frame, with sheet metal screws. Pan head #8 x 1″ should be long enough to work, and not penetrate to far into the case. It would then be even more secure then just wedged in place. Be careful where you place the screws into the AC case. You may have to move the cleat further back on the case to avoid hitting something you don’t want to hit, but it should still serve the purpose.
I’ve seen many great old windows butchered with AC installations. The instructions they come with almost assure this will happen with all of the clips and brackets that they say need to be screwed into the window stool, sash and frames. My system only puts 2 – 3 screws into the window sill and if you cut it to fit tight into the window sill, and are careful, it will be realigned with the old holes again when you reinstall it the next summer.