Airpods charging cases are very similar in size to the funkey at 44.3 x 21.3 x 53.5mm. Given their popularity it makes it very easy to find a nice case. E.g. MRPLUM Earbud Case Compatible with AirPods PU Leather Hard Portable Carrying Case Protective Storage Pouch with Mesh Pocket & Keychain for Wireless Headphone Earphone USB Cable & Mini Items (Brown) https://www.amazon.co.uk/dp/B07S8G2DN2/ref=cm_sw_r_cp_api_i_tt.iFbM4S6VDY
MoKo Case Fit AirPods 1/AirPods 2, Premium PU Leather Vintage Style Snap Closure Protective Cover Carrying Pouch Pocket with Keychain for AirPods 1 & AirPods 2 Earphones Charging Case - Dark Black https://www.amazon.co.uk/dp/B07TVSJ15C/ref=cm_sw_r_cp_api_i_yA.iFbG2RB64V