Here's an idea, it may or may not work out for you: I have an Oceanic BC, and my understanding is they're sister companies. The webpage for the Tortuga
Welcome to Sherwood Scuba! , as I expected, says one of the features is "adjustable cummerbund". I'm going by how my Oceanic Islander works; if I'm way off base on they're being similar, you'll know quickly.
I know it seems like a big step, but take a hard look at how the cummerbund is attached to the BC through the slots in the backplate. It's not rocket science, just a weaving in and back through a couple of slots, aided by some velcro for added friction. See the attached drawing, that's what mine looks like. It might be complicated by a spine pad attached via the same slots, but it's nothing that needs a Merit Badge in equipment maintenance, just straps in slots. Keep track of what you started with with a drawing, and you can put it back.
Without actually removing the cummerbund, loosen it a little at the strap end opposite the cummerbund. Then feed the slack through the slots to the cummerbund end. You may find you can make the cummerbund several inches longer at your waist, and remember there are two sides that can be adjusted.
Depending on where you bought it, your LDS might do this for you, but almost anyone ought to be able to do this themself.
If this isn't applicable to your BC, or if you can't get enough slack to solve your problem, sorry for the interruption. If it's the latter, having an extension sewn on the inner end by your local cobbler may be cheaper than any other solution.
If none of that works, try contacting Sherwood support (or your local dealer) and see if they have a larger cummerbund available.
