I'm going to post this as an answer, but please don't take it seriously because nobody will understand this, and you already have a good answer. The most concise way to write "mañana por la mañana" in Spanish is probably cras prima.
- adv. dem. desus. mañana.
- f. Una de las siete horas canónicas, que se canta a primera hora de la mañana, después de laudes.
- f. Primera de las cuatro partes iguales en que dividían los romanos el día artificial, y que comprendía desde el principio de la primera hora temporal, a la salida del sol, hasta el fin de la tercera, a media mañana.
So, cras is an obsolete way of saying tomorrow in Spanish, and prima refers to the first hour in the morning just after the down. But I am very much afraid that cras prima is ungrammatical, nonetheless.
I'm too much tempted to add the translation-golf tag to this question... :-D
Now for a more serious answer, I would write just amanecer or, using a shorter but somewhat poetic-ish form, al alba. You don't need to write mañana because the user will see that bit of information between the ones corresponding to esta noche and mañana, so the user will understand that amanecer refers to the next down between tonight and tomorrow.