I might approach this as a larger brand extension pitch. In order to get them on board your design work, you need to market them on everything else a business would be concerned with and alleviate any fears.
Exactly how does this fit in their product lineup? in their business model? What are the costs on manufacture? could you allocate same factory resources? retail price point. Is there something incredibly niche that fits their future marketing direction? Can they buy any patent rights? (ex: a unique closure system used in packs AND clothing)
Everyone above has great advice. I’ll add:
You are a business… but other companies will only buy in if the numbers add up. Most will not gamble on unproven tech/product/service.
Jeff