Hi, my first post on here.. so here goes!
I would seriously consider checking out "Camo Macs", just type Camo Rain Cover in ebay and you'll find 'em, from scotmoulds
I ordered a couple, one for my Sigma 120-400mm f/4.5-5.6 DG OS HSM and my mates using one on his 150-500.
They fit well...