An artificial lepidopteran hibernaculum, of course!
My guess was an overwintering box for ladybird beetles (ladybugs). Close, but wrong. A web search told me this contraption, installed in Paulik Park, is an overwintering box for butterflies. Inside there should be bark containing butterfly-sized nooks and crannies.
Two butterflies that overwinter as adults in these parts are the Mourning Cloak (link describes how they survive freezing temperatures), and Compton Tortoiseshell.