The budget for the movie - based on the cult teen drama - was raised by Veronica Mars fans via a Kickstarter campaign.
In a message to contributors, Thomas revealed that he has now completed a first script of the film, which will see Kristen Bell reprise the role of Veronica.
"There's a lot of work left to do, but it's a movie I can't wait to make," Thomas said. "I think it's also the movie you've been waiting to see."
The Kickstarter campaign launched with a target budget of $2m, ultimately raising over $4.5m, but Thomas urged fans to continue contributing to the fund.
"$2 million was our minimum goal," he explained. "It would be enough to get a movie made, but it was never going to let us make the exact movie we really wanted to make, or the one we know you deserve.
"We're at $4.5 million now. That's a lot of money. But for a feature length movie, it's still a pretty conservative budget."
The writer/producer added that more money would mean "more options", claiming that the scope of his script would be shaped by the eventual budget.
"More backing means more locations, more sets, more actors, and most important of all, more shooting days," he said.
Thomas previously hinted that Veronica Mars could continue beyond the upcoming movie if the revival proves to be a success.
> Veronica Mars movie: Jason Dohring joins Kristen Bell on film
Watch the video teaser for the Veronica Mars Kickstarter campaign below: