Stray Watchtowers

The Stray Watchtower would house a portal to the Icebound dimension, an earlier idea, and boxes of random loot.  It could spawn in any dimension but the Nether.

Boxes: A brown box crafted with 8 cardboard.  Would work like a barrel, but can be double-sized and have 9 extra storage slots.

Cardboard: Crafted from 9 paper.

The portal: It would lead to the Ice Fortress.  Each Ice Fortress would be made of frozen bricks or, rarely, frozen prismarine.  The Ice Portal could not be built or damaged by players or mobs, except Strays.

Large groups of (20+) Strays could construct Stray Watchtowers in twelve in-game days.