In theory there is no problem using a metal enclosure and connecting the GND to the chassis. You have to take care of the metal box making short circuits between pins... what do you mean with
ll the signal is drain out by the aluminum of the box.
There is no problem connecting pedalSHIELD ground to the metal chassis (enclosure), but if you are experiencing a signal drain when you do it is because there are some unwanted short-circuit or guitar signal grounding.
The most usual scenario is that the power jack, the guitar in/out jacks, the pins under the PCB are also connected to the chassis, creating an unwanted connection and draining the signal.