Why not just [name]Sam[/name]?
I know a boy named [name]Sam[/name] ([name]Samuel[/name]). His mother actually dislikes the full name [name]Samuel[/name], but fell in love with [name]Sam[/name]. Her family encouraged her to use the long form, even though she only ever planned on calling him [name]Sam[/name], the even now, the boy only ever goes by [name]Sam[/name].
I understand wanting a longer form of the name to have something more formal to fall back on though (I have the exact same issue with the name [name]Maggie[/name]…I [name]LOVE[/name] [name]Maggie[/name] but am not fond of any of the long forms).
I like [name]Samson[/name] best of [name]Sam[/name]‘s long forms. The only other “[name]Sam[/name]” name I can think of is [name]Samar[/name], which NB has in their unisex names category. They also have a [name]Samal[/name] in their boys’ list, which I had never heard of before I looked.