Well its not a westfalia made one but for £2.99 it fits a treat and its very stretchy and with two packs of bungee straps for £2.98 i can use the roof rack. all the hooks fit straight to the westy retainers plenty of room for loads more under it Add a few bungee straps for £1.49 for two and away you go , a good £6 spent me thinks All from the car bits aisle at the Range .
I have those very same bungees on my Westy...they fit a treat. However, I was a bit paranoid about putting my awning and camping chairs up on the roof rack and them falling off. Picked a couple of these up today from a local outdoor store... http://www.craigdonmountainsports.com/outwell-ratchet-tie-down I figure they would also be easy to get online. I've been looking around for some suitably small ratchet straps for a while and these fit the bill - and cheap too! The hooks fit into the westfalia retaining hooks. Obviously I'll need to take care not to over tighten them! For £3.19 each I bought a couple!
Even cheaper here! http://www.outdooraction.co.uk/camping-tent-accessories-camping-extras/outwell/outwell-ratchet-tie-down-pd-4238.php
I bought a strap thing like this for my roof rack for £6. It lost its stretch and fell to bits after a year.