I have installed new diaphragms in my Ecoplus pump a few times over the years and have made them from various material, some good, some notso.
I try to find a good rubber, cloth backed material. Gloves have been a good source for this and finding ones that are a good thickness, flexibility, and very flat has been the challenge.
Today I bought some blue gloves at Lowes that are great. They are PVC and have a slightly sticky feel.
Item #184258