Practically anything will work reasonably well as a "receive only" antenna with that radio.
Back-in-the-day, using an S-108 as an SWL, I just strung-out a 50-foot "random wire" under the eaves of my house up about 20-feet. Pulled in lots of signals from both shortwave broadcast stations and ham operators.
So your "Slinky" in the attic will probably be fine.
Some other easy-to-build options can be found here:http://www.hamuniverse.com/shortwaveantenna.html
All that said, you'd definitely do better with something outside, if that's possible.
Mel - KS2G