This Cache
The cache should be fairly easy to find. You're looking for a Film Canister, so don't forget a pen or pencil. This cache is on the southern side of Shepherd's Lane. Do take care when walking along here as the cars go quite fast; I witnessed a near miss between a car and van whilst replacing the cache recently.
About Roosta
From Wikipedia:
Roosta is a hitchhiker and researcher for the Guide, whom Ford Prefect knows at least in passing and holds in some regard (Ford describes him as somebody who "really knows where his towel is"). He carries a special towel infused with nutrients, wheat germ, barbecue sauce, and antidepressants, which can be obtained by sucking on different areas. The last two of these, he explains, are for use when the taste of the first two sickens or depresses him. He saves Zaphod Beeblebrox from a horrible death in the offices of the Guide (by taking him into the artificial universe in Zarniwoop's office), and is then kidnapped along with Zaphod and the left-hand tower of the Guide building by a squadron of Frogstar Fighters.
Congratulations to Eh1414 for the FTF!!