Another possibility you might run by your lab -- black ABS drain pipe with cement type caps makes very nice film storage tubes. The 1 1/2" size will accept a 4x5 with the small dimension curled around the inside of the pipe, and with a little care you can insert several sheets in a tube. The caps press on easily, are light tight and weathertight (watertight, if they're on tightly), and can be removed easily with a twist to break the friction. You can make these with a handsaw or hacksaw and sharp knife to debur the cut ends, at a cost of around $5 for ten feet of pipe (will make a couple dozen tubes at 5 inches each) and around $1.50 each for the caps.
Check with the lab first -- ideally, take in a tube with no film to show them so they'll see what's up, and clearly mark the tubes for content, ownership and disposition. They're a lot cheaper than film holders, and IMO more secure than a film box -- but there may be labs that don't want to deal with them...