Kris-

Same thing happened to me, and the best I could come up with was a, brace
yourself, urinal plug.  From the plumbing supply house...an inch and a half
long rubber plug with metal washers on both ends, and a bolt through the
middle with wing nut.  Tightening the wing nut compresses the plug, causing
it to expand outward to the sides of the downtube.  Holds on suprisingly
well...didn't require much tightening and kept a water tank full over a 300
mile weekend.  If you find a source for the real caps, let me know.

Andy
84 Westy

I'm trying to jury rig a replacemen until I can score a new one.
 Does anyone have a supplier or source for replacements?
Kris Weber
86 Westy Syncro {F.S. at the right price}
98 Jetta TDI
98 Cabriolet