[Aw shucks~] Thank you, young man. That is kind of you to say.

If you wish to add mayonnaise to the recipe I provided, it would likely be best to use it in place of the butter. The roux's base is some sort of fat mixed with flour, which acts a thickener. Of course, having not made a roux with mayonnaise before, this is...something of an educated guess.

Do be sure to take Kagami's recipe into account as well. He runs a diner, and having tasted his food on many occasions, I personally advocate for his culinary talents.