I did the design in HeeksCAD. It's basically just an elongated cube with a sphere subtracted and the corners filleted.
The spherical pocket is to hold the "pressure vessel". I'm using a bottle Coke put out for Christmas last year. PET bottles are rated to hold 200 psi but I never pressurize above 120psi.
After cutting it on the CNC router and sanding:
There's also a couple routed pockets on the bottom for the battery, switch, and wires.
The magic is done with this solenoid valve. Model 35A-AAA-DGGA-1BA
The valve can be connected in different ways. I've got it set up so that when current is present, the valve is open. The switch in the front cuts off the power and closes the valve.
So when the car is on the starting line, the switch is pushed closed by the gate, holding the pressure. When the gate drops, the valve opens.