I actually made wedge shaped pads that I put on my stirrup flat , in order to help my feet NOT roll outward. I have the outside part of the bar a bit higher than the inside. I did this using Cashel stirrup pads, and putting some pieces of felt under them in such a way as to build up the outside verses the inside. because my stirrups are on the large side, I can get away with this. if the stirrup were tightly fitted to your foot, adding any padding would be a no-no. I did, however, remove the rubber foot bed thingy first, before creating my wedge shaped pads.
you can buy already wedge shaped foot beds to exchange for the normal flat ones that are inside a Fillis iron. you might try this as an inexpensive possible solution. I just spent 5 minutes trying to find the ready made ones online for you to buy, but could not find.
I know that there are gizmos you can buy for western saddles to help the stirrup hang at an angle that is flat for your FOOT, not flat with relation to the ground. meaning, the outside is a bit higher than the inside. this makes things much easier on your knee, too.
experimenting with this positioning of the stirrup bed may help a lot.