So I’m trying to get to [name_u]Matty[/name_u], but I’m not a huge fan of [name_f]Matilda[/name_f], which is the usual way to achieve it. Can you think of any different names to get the nickname [name_u]Matty[/name_u]? And do you prefer [name_u]Matty[/name_u] or [name_u]Mattie[/name_u]?
I love [name_f]Mattea[/name_f]. I prefer [name_u]Mattie[/name_u] - it looks more classic and old-fashioned, somehow.
Edit: Amata might work. I think it’s gorgeous, but more of a stretch.
I was going to suggest [name_f]Mattea[/name_f] as well 
I also prefer [name_u]Mattie[/name_u].
I know a girl that goes by [name_u]Mattie[/name_u] with the full name Mattilyn. Also I know a girl who spells it Mattaline.
You could spell it :
Mattalyn
Mattalin
Mattalyne
Etc…
These are some great suggestions. I don’t think [name_f]Amata[/name_f] is a winner, [name_f]Mattea[/name_f] is interesting though.