MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Ideally you'll want to print it in a flexible/semi-flexible material (i.e. TPU). If you can't then it would need to be installed prior to assembly of the catapult. If it's an after thought then heat the part up with a hair dryer or heat gun (get a hot pad mitt and protect those fingers!) and slip it on before it cools.