Perhaps a automotive car seat - some 'recycling yards' have cars which are perfect aside from front end damage.
I would not recommend trying to build the seat yourself using padded elements. Chairs are simple in shape but very complex in how they work with the human body. I was in a woods class in college and we had to design and build a piece of furniture. A friend built a chair- it looked great but you could only sit in it for a couple of minutes before having back pains. Fast food restaurants have special chair/bench designs which are intended to only be comfortable for about 20 minutes, then you feel like you need to leave. Since this is going to be the centerpiece of your office/bridge and you are going to be in it a lot, basing your command chair on something designed for comfort over a long period of time would be the way to go IMO...
I would not recommend trying to build the seat yourself using padded elements. Chairs are simple in shape but very complex in how they work with the human body. I was in a woods class in college and we had to design and build a piece of furniture. A friend built a chair- it looked great but you could only sit in it for a couple of minutes before having back pains. Fast food restaurants have special chair/bench designs which are intended to only be comfortable for about 20 minutes, then you feel like you need to leave. Since this is going to be the centerpiece of your office/bridge and you are going to be in it a lot, basing your command chair on something designed for comfort over a long period of time would be the way to go IMO...