One thing no one has mentioned about the flex tank is why it is!
To install this tank it is possible to run your fuel tubing through the fire wall and then suck the air out of the tank and make it very small. Then put the tank in place and cut your fuel tubing to allow the tank to expand to original size and shape.
I have a Pattern plane that this is the only way that you can change out or repair the tank. If it wasn't for the flex tank you couldn't service or replace the tank..

Just my $0.02 worth


Don