Hint #1: It's a box about 35 cm tall, 20 cm wide, and 8 cm deep.
Hint #2: It's got a piece of wire mesh (hardware cloth) nailed near the bottom edge, against the back panel.
Hint #3: It's available commercially.

A. A vitamin "lick" for deer, used in areas where deer reportedly suffer a health-threatening deficiency in Vitamin A

B. A "night registration" board used in campgrounds and RV parks

C. A Mediterranean fruit fly trap, used in areas where these insects cause crop damage and need to be counted to determine the level of infestation

D. A "bat house" (like a bird house, except for bats)