Check with Ames and see if their ramp is going to be opened this year. They were shutdown but the town last year but I've heard the worm has turned this year for them.
West Alton marina indates that you will probably fish the southern half of the Lake so try Roberts Marina on the East side almost directly across the lake from West Alton.
Alton Bay also has a public ramp but the parking is a bit "involved".
From there I think you would have to going more north toward Fayes or some of the Weirs Beach/Paugus launches.
You should pick up a Bizer map of the lake. All launches are marked on it and it has all the info anyone who navigates the lake should have on a map. Worth every penny. Paugus and AJs both sell them.
you can also consider Shep Browns out on Merideth Neck but I think they charge a fee even this early in the season ($20) but I love the launch and the access to the Broads.