A couple of ideas, Malana.
"Cable grommets", all explained here...........
http://www.maplin.co.uk/p/maplin-hole-plug-grommet-95mm-10-pack-rg78k
The trick is to get the right size.
The other thing is to use a chrome or stainless steel bolt with a domed head on the bolt .......... a "coach bolt". About 2" (50mm) length should do it. For a 12mm hole, an M10 bolt should do it (M10 is the size - 10mm diameter)
Here's a 60mm one.............
http://www.screwfix.com/p/threaded-coach-bolts-a2-stainless-steel-m10-x-60mm-pack-of-10/63369
A little smear of silicone sealant under the head will keep the water out.