What kind of materials can I use if I want to put an extra layer under the soles in my shoes? My right foot is slightly smaller and sometimes it gets frustrating to find the perfectly fitting shoes.
In the past, I have used pieces of cardboard to raise the soles a little but wondering if there is a better/cleaner way. Thanks in advance
It’s depends. If you need to reduce whole volume any hard foam volume reducer will work. Seek for superfeet or sidas. Also you can buy sheet of any hard foam and cut it. In other cases you can use heel shims in (maybe) combination with tongue pads. This approach helps to deal with shoes with roomy midfoot.
Another option is to use shims in between the cleats and the shoe. With foam or an insole, it might get pretty crammed in the shoe.
How does this help with different size feet?
Perhaps I misunderstood the original post: the OP wrote that they wanted to put an extra layer under the soles of their shoes. Under, to me, suggested this was a problem of leg length.
He’s trying to fill the inside of his shoe for a smaller foot.
Why would you put something under the sole then? Using special insole would make sense to me, but I don’t see how putting something under the soles would help. 
Yeah, maybe a thick wool sock?
Because I like the sole I have now and all I want is to reduce the volume around one of my right foot like @Aleksandr_Dorofeev recommended.
Thx, that works (&worked) in the winter but not too practical in the summer.
If you like your current insoles, then maybe the putting cardboard under the insole really is it.
Insoles do differ in volume. I am pretty sure Superfeet make a bunch of different volumes and arch heights. I like Icebugs myself. The yellow insoles are really thin. The blue ones are a fair bit thicker, and I’m not sure how much. Both versions come in 3 arch heights. Problem is that you can’t really buy single insoles, so this option would have you doubling the amount you spend on insoles.
Cardboard is a terrible idea IMHO, it will go soggy and make a mess. I would aim for any thin flat foam inlay (think odour eaters, not orthotics). You could get some 2 or 3mm EVA foam sheets from a craft store, but any decent shoe shop should have something you could use. Just make sure it doesn’t bottom out under a light squeeze if your fingers, or it won’t last long. Poron is another good material often used in footwear and orthotics for adding cushioning or reducing volume, but I don’t know where you can get it in small amounts.
How have you dealt with this your entire life? For many of us, this would be completely foreign, and our advice would be moot. You’ve had to figure out a way to adjust the comfort your entire life and I would imagine that you have more insight.
Personally, I would just tighten down my shoes as much as possible while still being comfortable, and get used to the slight extra toe space on one shoe.
This is good stuff. Thank you.