If you'd like to have more control over how you contribute to the ceremony, you can contribute in OFFLINE/CUSTOM mode. You'll still have to run an "empty" client that signals to the server that you're indeed in the process of contributing (else the server will skip over you - see "Notes"), but the actual work of generating the contribution can be done by yourself.
This directory contains scripts that will help you download, compute, and upload your contribution. Documentation is in the "Run a client (custom / extra security" section of the participant guide.