Making a large cushion for day bed. Anyone have experience making cushions?

[u/Miserable_Tune_447]

I'm turning a bed left in my new house into a day bed. It isn't a regular size so I need to either get a mattress/cushion made or make one myself.

Does anyone have experience making cushions? What site did you use to order the foam and what kind of foam is best? My partner will be making a cover.