Use a metal electrical/project box with a robust momentary switch.
Valid approach, IMO. If you want it to be more of a pedal than a switch, maybe some wood/glue and a pin or two could make it feel that way for no real cost. I imagine auto parts stores here would have a heavy duty switch that would work very well. Almost all the racecars from my early days used a 'Ford solenoid' (Ford starter solenoid powered by 12V that was good for 100+ amps used on a bunch of Fords from the 60's-80's, IIRC) and a sealed/hard toggle pushbutton for starting. You wouldn't need that ampacity, so a simple relay would be fine, but the concept is the same.
I thought sewing machine pedals were analog and not digital, so it would mean you needed to slam the pedal to make sure you don't get a slow build of current until the relay closes.
I still like the idea and hope to see some versions for solo launch guys, but playing Devil's advocate, you'll still move your body a bit when pushing the pedal. If the goal is to get focused on the rocket with your camera, would a better (solo only) option be to use a form of timer switch? You'd get the picture framed and generally focused, flip the 10 second timer switch and then do final framing/focus tweak and then the rocket flies based on the timer input. Making a 1 second beep and/or having an abort foot switch could be features.
Another thought would be to use a dead-man switch to initiate launch. Basically, hold your thumb on a normally closed switch to open it, arm the pad with a normally open switch, frame, focus and then slide your thumb off the dead-man switch with minimal body movement.
Not sure if any of these would be valid for a group launch and/or violate the rules, but just wanted to throw them out in case the end goal (getting good footage) could be done in an alternate way at a solo launch without causing any safety risks or requiring a specific hard to get component.
Good thread!
Sandy.