If you've got a Tamrac bag (or anything with horizontal loops at the sides) then you can expand it's capacity with one of these.
Like the other MAS system cases, there's a large 3" wide flap at the back with Velcro and press-studs to ensure that the case will never come off.
The case is well padded, and will hold a lens up to 7" maximum. I can fit a Nikon 18-200 & 35mm together, or a Sigma 150mm, for example. The only slight problem is that it only has a zip to keep it closed. Maybe you wouldn't need regular access, but a Velcro or clip fastener would have been a nice addition.
I find Tamrac cases and bags are always of a good quality, and weatherproof too.