Here is one original accessory for your iPhone. It’s a speaker amplifier in the shape of a horn.
Best part of it is that it doesn’t need any batteries as it takes advantage of the horn to naturally amplify the sound coming out of your speaker by adding up to 12 decibels. That’s what the product description says anyway.
You can get this amplifier for $25 from here. More iPhone accessories in our very own store (support the site!).