Why not just make an iPhone case that has a carve-out for the payment card? That way you can use your payment card and not need your phone, too (like going outdoors, to the beach, etc). I ask because I've already seen iPhone cases like this in Japan. I use one with my Clipper card.
I too have seen this kind of product in Japan. The downside is that the case gets thick, the design is limited, and it doesn't look cool. As a former user for using Suica app of feature phones, the case product is apparently not optimal.
IMO users who put their phones in their pockets would gain the most benefit (mostly men I think). Women, especially in Japan, generally wear clothes that have no pockets and the phones are inside the handbags, which will have less benefit. It is not much different taking the card case or a phone from your handbag.
I have seen a brand handbag that has an inner pocket in the bottom to hold a Suica card. To use it, you just put the handbag on top of the ticket gate. This kind of product looks more suited to women in Japan.
