K bike side cases (a.k.a. System Cases or Integral Cases) utilize plastic hinges (part 46542300039) that wear out and break over time.

It's annoying when they break and they cost $8 each to replace. Aluminum piano hinges make a better, stronger and permanent replacement.

There are probably other sources but I get my piano hinges from McMaster-Carr: https://www.mcmaster.com/1581A281/ 

For cutting metal it is handy to get a metal cut-off wheel for your table saw. You can find these at Amazon, Harbor Freight or your local hardware store. An example is the Dewalt DW8005:

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