The idea for using a cheap lens (not necessarily plastic!) came from this website:
http://galactinus.net/vilva/retro/eos350d_meniscus.html
I bought a number of cheap (positive, meniscus) lenses from Anchor Optics (New Jersey, USA) and found some old toy cameras from which I scavenged lenses. I was also able to pick up a film camera (from a thrift shop) which had a reasonable focuser still intact and constructed a housing for the lenses which mates the focuser to the Canon camera body. The construction used a Canon EOS "reverse mount" adapter (couples a lens filter thread - male - to Canon EOS mount) which came from eBay and I adapted the focuser to this reverse mount. There's a skylight filter between the focuser and camera body to protect the ccd from dust. At the website above, an EOS to M42 adapter is used which can also be purchased on eBay. For a given lens, there are a range of focusing positions that can give a variety of "Lomo" effects. Basically it's a digital Holga camera and good fun to use.