Hi Debbie Yes, it is that time of year! (smile) Although we have a good central A/C, I have trouble sleeping, thinking of the hugh amount of electricity flowing through it at nite, when the only space we occupy is our bedroom. So we too have installed a window unit that does a great job in keeping our sleeping area cool at nite while giving our central unit (and my mind) a breather.
What I do is take an old 3" foam pad, put it on the window sill and set the portable A/C on it, then bring the sash down on top of the A/C. I stuff fiberglass insulation tufts on both sides to seal out outside air. However, your casements are going to make this a little harder. I would make sure and do some width measurements to make sure the portable unit will fit in the opening. You might even consider taking the window out, inserting the portable A/C and installing a plywood cover (or framed glass panel if you want to get a bit more complex) in the window space above the A/C unit. This will take a bit of carpentry skill.
One other thing....make sure you have a wall outlet with uninterrupted 15 amp service to it. We've found out the hard way that having more than one electrical appliance on the circuit with the A/C always seems to result in blowing breakers. And if you have to use an extension cord, make sure its rated for continuous 15 amp service.