I've already looked for such a case here on Thingiverse but unfortunately couldn't find one, so I simply designed one myself. It is a housing for one of the cheap ESP-01 Relays which you can buy on Ebay or amazon. Since I am still a beginner with 3d printing, it took a while until I got a good result. (Seen here in blue.) The housing has a lid which can be clicked onto it.
ESP-01 Relay Lid , 100% Infill needed, no supports. ESP-01 Relay Box , 25% Infill , supports for the Cable Holes.