Call your local GOP offices. However, it's possible that it's too late to sign up - poll watchers have to be approved by the Supervisor of Elections (that is, it has to be checked that they are validly registered voters of the party they are representing). In Florida, you have to have signed up at least two weeks before the election.
But one thing you can do at any time is call your GOP office and find out if they are waving signs on election day, going door to door to get out voters, calling voters, or even driving elderly people to the polls. These are all things that will need help, and your presence will also provide confidence to GOP voters going to the polls (since in some places, voters have to run a guantlet of Dem "activists" lined up around their polling places).
Crap. It sucks being a procrastinator...