Is there such a device, like a food cage or something, that keeps larger fish out, but allows smaller fish to go in and eat food that is left in it?

I would like my otos to have their own access to their own food to monitor how much they are eating, but the tiger barbs compete for their food. If I drop algae wafers, the barbs eat them as well, even at night in pitch dark. I know this because I set up a wifi infrared camera to watch my tank at night (hoping that feeding the otos after lights out would keep the barbs from eating), but sure enough the barbs were eating the algae wafers throughout the night.

Ideally I want to put algae wafers (or blanched zucchinis/veggies) in a cage or box of some sort at the bottom of the tank that isolates them for otos to eat. If nothing like that is commercially available, I was thinking a small plastic in-tank breeder set up upside down near the bottom of the tank with just enough gap to let the otos in might work. What do you think?