Resources for or about writing letters?

If this post doesn't belong here, feel free to delete and I'll post it in the simple questions tab.

I'm going to a convention soon where there will be some Japanese voice actors of a few different anime, as well as some Mangakas of a couple series I enjoy. There will definitely not be enough time to say what I want to say (as I will probably be super nervous to even speak in English, let alone Japanese), but I read that I will be allowed to bring them gifts and letters.

Since I wouldn't know what to bring, I wanted to write them letters to take with them thanking them for creating a lot of cool content that I enjoy. I know they probably won't read them, but I still want to show them my gratitude. However, I have zero idea what the proper style or format is for writing letters in Japanese. I've seen a couple of links here and there about it, but most of them are about writing a formal business type letter, or learning Hiragana. Basically, does anyone have any resources for writing casual, relaxed letters? Any guidance would be a help. Thanks!