I would also make the case a little bigger as the foam inside can be added to or you are able to use a sofa cushion refill service. I have recently had ,y sofas done which obviously isn't the same they did however say they add a half inch to all sizes if they fill them to make sure everything is nicely in the case. I got mine done from here if it helps however we all know the leather may expand so its more important to make sure you have extra foam in there for the long term. As someone else said you could just buy the foam sheets to add to make it filled in in the future. However as i did a cushion for someone i just used this sofa cushion replacement service which took away the hassle.
here's the website i used:
eFoam
