It looks like you’re referring to the New Montana Fly Company (MFC) Poly Box Leaf Insert, which is designed to double the storage capacity of the MFC Poly Box.
Key Features:
✅ Doubles Capacity – Adds an extra layer for more fly storage.
✅ Compatible – Designed specifically for the MFC Poly Box.
✅ Durable & Lightweight – Made from high-quality polypropylene.
✅ Secure Fit – Keeps flies organized and prevents spills.
✅ Versatile – Ideal for nymphs, dries, streamers, and more.
Why It’s Useful:
– More Flies, Less Bulk – Store twice as many flies without needing a second box.
– Better Organization – Keep different fly types separated yet accessible.
– Travel-Friendly – Compact yet high-capacity for guides and anglers on the go.
Would you like details on where to buy it or how it compares to other inserts?