A Bingo Marker works great! They have foam tips with some type of membrane backing for the ink/paint to ooze/squeeze out of, but I have found that if you poke a push pin through the membrane a couple of times you will get a better flow of ink for letter-writing.
The ink is thinner than poster paint and usually a lighter color so you may have to go over the words several passes; and it takes several minutes to dry (dry them flat).
They work great, make even thickness letters, and you can find them at Dollar Stores everywhere!
Go Get 'em, IG!