Dog activated food dispenser

Any recommendations on a food dispenser that my dog can activate? This is the first dog ive had that eats sporadically, whenever he's hungry, throughout the day. So, his food often just sits in his bowl for long periods of time. I didnt mind this at first but its beginning to attract bugs and other pests. Almost all dispensers I see require me to activate it remotely or set a timer and I'd rather train him to be able to use it when he's hungry.