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I would use your full maiden name in the middle spot.
I would go with how its spelled in your name, Solais. However, using your full maiden name would probably be fine just the same. I like when people honor their heritage, with a twist or using a name in full. Either way, I think its a great idea.
I really love [name]Soleil[/name]. It is beautiful and reminds me of the spunky girl, Punky [name]Brewster[/name]!
Thanks for the opinions everyone! 
Anyone else?
I would probably use the full last name. Only using a part of it makes it seem less substantial.
If your maiden name was particularly challenging to say or spell, I’d say go with the homonym. But in this case, Solais looks just as graceful and intuitive pronunciation-wise. I’d say Solais.
I agree with this. If it was a first name, I might go with the one less challenging to spell/prounounce. But as a middle, I would use the full name without alterations.
Waking this up to get some more opinions!
As someone who plans on using only part of her maiden name as a middle if we ever have a son, I don’t see the problem with it. I don’t think it makes the reference any less substantial. I would use the same spelling as is used in your name, but use whatever part you feel is best.