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Calb3rtstiffer springs for trampolines are harder to come by, you will have to find an old one and salvage the springs from that. they certainly don't make trampolines like they used to. but as mentioned above, you can add more springs from another trampoline. though i do not recommend criss crossing them because they won't last as long as if you do something like this...
l/l l\l l/l l\l l/l l\l l/l
just try out different combinations until you get your desired bounce. you don't need to do every single one because then it will be super stiff.
or if you have two mats the same diameter you could try putting them together and see if that works? i've never had two tramps the same diameter so i haven't been able to try it.