Hi Christopher,
Unless I missed it, doesn't look like folks suggested you visit
http://www.artglassanswers.com
It's a great site for all kinds of saftey and set-up related info. I'd go to the trouble of getting a log-in account so that you can see the entire site, including photos and drawings.
Since you were considering an enclosure, anyway, if you look under Studio and Ventilation, there is a thread on an inexpensive fume-hood style ventilation enclosure you might find useful. One of the variations discussed incorporates a box fan in a window, very similar to your present set-up. Looks like you could easily build this enclosure around your currently installed fan and add yet the next level of safety to your present temporary configuration for a very modest cost (foam board and tape for less than $25??). It also looks like you could add this to your current set up in less than a couple of hours - looks really easy. That said, I would encourage you to upgrade to a better fan/installation system just as soon as you can, as other folks have recommended. I've run across mention that outside winds/breezes can sometimes cause back-air flow with the box fan-in-window approach, which would obviously defeat the purpose.
The thread mentions incorporating a baffle at the top. If you can possibly afford it, I would make this out of plexiglass or regular glass, whatever you can afford, so that you can see through it and have it extend low enough to use as a face shield. This will give you even more protection against fumes.
The only downside about the idea in this thread the fact that the foam board is flamable. There are ideas posted to make it safer that I'm sure you'll want to consider, similar to some of the advice posted here. If you're handy and have a few basic woodworking tools, you might want to make this out of plywood, instead. The cost to build should be similar.
If you search the Art Glass Answers, LE, WetCanvas and ISGB forums, you should find lots of ideas for other good, inexpensive enclosures, wood and otherwise, like the one proposed above and some of the ideas in this thread. Michael Barley, a fabulous artist, has what looks like a very good design for one. If you want to search for it, I think I've seen it called the Barley Box.
Best of luck to you
Linda