I've recreated those caps for the Simjack pedals elastomer and i think you will love them.
You have to put them on top and bottom of elastomers and the raised edge will prevent them from expanding/squishing out much when pressure is applied on the brake pedal. This will increase the feeling of the pressure that will be more progressive and stable.
I've made out 3 model and the only difference is the high of the raised edge so you can play around with it. Obviously more raised mean less expansion and more stiffness.

-3 mm
-5 mm
-7 mm

The standard dimension is:

30mm external diameter
25mm internal diameter
5mm thick raised edges
10.5mm center diameter for the rod

I've used PLA and work perfectly, i think PETG will be a bit better because is more rigid.