Hi,
Those rubber band thingies pointed out by WRe, IMHO, will do nothing.
That spring material shown in his photo, however, is used extensively in our seats. It's a common material called "nosag" or "zigzag" upholstery springs. You can find it at most online upholstery suppliers, even Amazon.
When I took my seat apart, I tested it and I "bottomed out"; the bottom of the seat (the springs) hit the floor. So, I bought new spring material--heavier gauge than factory. I "siamesed" additional spring material onto the existing springs. I added additional rows. I added cross rows. When I was done, I could barely budge the springs with my hands. Then I sat on it...and it bottomed out. It bottomed out when my lightweight daughter sat on it. Clearly, the springs were not enough.
Enter the pool noodle. Basically you take a cheap pool noodle and bend it into a C shape; you'll need to do some cutting. The back of the C fits at the front of the seat where the springs form a coil. The legs of the C taper back. You'll need some cutting, but don't worry--a pool noodle is the least costly thing you'll ever buy for a Pagoda. Cut and carve and stuff it into place. It firms up the seat quite a bit.