I use the Plano boxes for my warm water flies as well.
Many years ago I learned a valuable lesson when for 2 fishing trips in as many days, I forgot my reel at home
Ever since then, I've been a lot more organized with my gear.
I have a trout vest, a saltwater vest, a warm water boat bag, a salt water tackle bag, and a warm water shoulder bag for wading.
Each bag or vest has it's own leader wallet, tippet, appropriate tools, terminal tackle, fly boxes, etc.
In other words each one has everything that I'll need to just grab and go.
What's my point ?
If you're just shoving things in your pockets when you head out to fish, sooner or later it's going to bite you in the ass
For my warm water tackle bag, I use an Swiss army surplus gas mask bag that I bought years ago for 50cents.
You can still find similar bags for 12-15 bucks. Well worth it, I think.