Century/Schneider sunshade/filter holder
 

Anybody ever used one of these?

WIDE ANGLE SUNSHADE/FILTER HLD - Schneider Optics

It's not as cheap as it looks at first glance...$195 for the basic thing, $95 for the adapter ring, and $155 for the filter tray. Light and no rails needed, but only one filter slot.

There's also the Lee bellows:

LEE Filters | Lens Hood - Wide Angle - with TWO Filter | WALH2S

Which is about $350 with adapter ring, and it has two filter slots. Back in my film days we always used these instead of matte boxes. Lightweight and flexible, no rails required.