Make:

Have you considered a good pair of heavy, sound deadening curtains or drapes? Those may be more effective.

The sealed plexiglass approach will give you some deadening of sound. The only question will be how much, and whether any variations could produce more sound deadening (e.g., three sheets of plexiglass instead of two). Obviously, putting a vacuum inside the sheets would give you even more deadening, but trying to suck all of the air out of an object with a surface area as large as a window pane will be problematic at best (and, downright dangerous at worst!). However, if you can make a (somewhat) gas tight seal, it may be worth flushing the inside with a light molecular weight gas (e.g., Helium), except that it'd be darned hard to contain Helium.

In a former life, I was a recording engineer. In fact, (next bit added for credibility, not for ego - I would rather be known as a janitor than music business jack@$$.) I worked on music row in a pro studio. Expertise questionably proven; on to your question.

Dave's suggestion is a pretty good one. A cheaper solution would be to go to Goodwill and grab some old sleeping bags. Just hang them when it's time to track and I think you will be surprised how much they help. As far as the Plexiglas idea, I would avoid it. The reasons range from quality of isolation to payoff for hours spent. Let me offer a few solutions you might not have thought of.

When attempting to reduce noise for recording, the first step is listen critically to a test recording done without improvement to the room. As you described your desired set up, I have to imagine that the majority of your recording is done with a dynamic cardioid microphone. (Example: Shure SM58) These mics pick up sound in a rounded heart shape, actually its more like a tomato shape, and do a poor job picking up sound that come from behind the diaphragm. Which is a good thing! As an experiment, set the mic on a mic stand, with the body parallel to the floor. Sing a bit, then rotate the mic 45 degrees, then repeat. See what mic placement yields.

Outside noise that enters a recording often travels not through the air to the mic but through the floor and up the mic stand. Any anti-vibration isolation you can do here can't hurt. Start with placing a mouse pad or two under your stands base. If you can afford it, you might think about getting a shock mount, or I'm sure you could build one. In essence a shock mount is simply a microphone holder that uses elastic bands to hold the mic. (My apologies if I am talking down to you, I am just trying to be clear.)

I would like to encourage you to embrace your noisy environment. Too much isolation results in a sterile sounding recording. When I was recording, we would track in room with an open grand piano, drum kit, and amped bass at the same time. Those demos were not only good, you no doubt have some of the final recordings on your ipod.

Lastly, If you are creating music for yourself, then don't sweat the noise. If you are recording song writer demos, worry most about the song. One of my first jobs as an intern was to pre-listen to demos, producers get a ton of them, I never threw any good songs out on account of noise. I did once see a tape thrown away because of the full nude 8.5 x 11 that came with it, so avoid that.

Good Luck

What about just getting a sheet of insulating foam from the hardware store? You can cut it to precisely fit the window so that friction keeps it in place. Maybe wrapping it in cloth and/or hanging a quilt over the whole thing will help, too.

@hartleroad

Nice comments. I remember researching microphones and noticing those signal pickup shapes. (They look very reminiscent of antenna propagation shapes.)

Great advice. Those little details seem like they could make a big difference.