Commerce: Unique single products + Custom line items, How to handle? I am building a shop with several types of custom cushions available. Each product (cushion shape) will have different options therefore i am using custom line items.

The issue is that to me it seems i need to create just as many different content types (product display) in order to reference the custom line item in the add to cart form of the Display?

There must be better way to do this?
Unfortunately there's not, you need a content type to match every line item type if you're dealing with different attribute fields. 
That was the original intention of Commerce custom product but it's at the point of providing different product types, but the feature hasn't been developed yet.