Well, there are no cats in the Tanakh, but the rule that before a word whose first vowel is a nuanced shva, a ḥaṭef like ֲ / ֳ / ֱ , the prepositions לְ־ כְּ־ בְּ־ take the equivalent vowel: כַּאֲרִי like a lion (like the lion would be כָּאֲרִי). The most interesting case is בָּאֳנִיָּה, which means in the ship, if pronounced bāʾŏniyyâ (i.e, Qamatz Gadol), but in a ship, if pronounced bŏʾŏniyyâ. (i.e with a Qamatz Qaṭan).
But you are creating it. Designing doesn't mean creating an object. In the case of knitting, designing would be designing/creating the pattern. Fashioning it, is for instance, used if you say have an old garment you no longer wear, but update it so it's wearable. Or use accessories to create a full look. Fashioning clothing is creating, designing is the blueprint for the creation.
Well, for phonetic reasons you pronounced traditionally the form without article לַחֲתוּלִים to cats too, because the first consonant of the word חֲתוּלִים cats has a חֲטַף פַּתָּח, a Shva with a-colouring, to which the Shva of the preposition assimilates. Colloquially you are right of course.