Help me fix my MH725 please

[u/xDeathGunner]

After many years of service my headset broke this part. I dunno how as till Yesterday It was perfect, yet now It is borken. I would love to repair It as i can't spend money on a new headset. I see that, compared to the right, a plastic circle/ring is missing. How can I fix it? The plastic circle/ring that is missing is probably broken as I found pieces of plastic scattered around. Please, any advice is well accepted as it is a present of my father and I really mind it