I have been using a filofax as a wallet for a few years now and have never looked back.
As they were starting to get a bit tatty after 3 years, I decided to upgrade my wallet credit card holders in my Filofax mini Cavendish in calf leather.
The card holders open from the inside so the risk of cards slipping out when the filofax/wallet is open is reduced considerably.
I use this as my permanent wallet. I picked up some cheap but strong credit card holders at Officeworks which has 10 pockets for four dollars.
I easily pulled the credit card pockets out with my hands and then removed the two studs with pliers (ok scissors, but don't tell mum).
To make it even hardier, I decided not to cut the pockets individually but to place it totally into my wallet. This created a bit more stability and should stop each sleeve from tearing from overuse (especially the VISA pocket).
I marked the holes from my previous card inserts as they were exactly the right distance and position.
However, putting the holes in required a hand-held hole punch, and herculean strength from me. Luckily my Australian rugged manliness came into the forefront and I was able to do it. (Ok, I got a mate to do it...but he does work out).
As you can see, it fits back quite nicely in the rings, and I am very pleased with the result.