For a really cost effective and secure option, Nalgene makes
Snap Cap Vials, in multiple sizes. These vials are far less costly than many of the battery locker suggestions above, and are almost as waterproof as the $20 per unit locker options. The small, medium, and large
Snap Cap Vials are sized to easily accommodate 16340, 14500, 18350, 18500, and 18650 cells. (No known Nalgene options for 21700 cells, however.) I've used these to isolate and protect my cells during travel, backpacking, and canoe/kayaking trips; the snap caps are tight-fitting, and water has never penetrated the enclosures even during heavy rain, nor when exposed to the inevitable splashing encountered during self-powered watercraft adventures.
Prices vary a bit, depending on the vial size, but these Nalgene vials start at about $1 for the smallest size (16340 compatible), up to $4 for the largest (i.e.- 18650) size. Search online for
"Nalgene Snap Cap Vials" and take your pick of several online vendors. Give these a try: you won't be disappointed.
Here's a photo...
(I'm not affiliated with Nalgene in any way, I just want to pass on what I consider to be a really good option for community consideration.)