I recently purchased an
All-Ett Sports wallet. It's extraordinarily thin; much, much thinner than the clamp I used to use, and definitely thinner than the Costanza wallet I was rocking years ago. I disliked my clamp because it didn't hold my cash and cards well enough and it still had a thicker profile than the All-Ett.
I keep about 4-5 cards on me (2 CCs, DL, Work ID, Insurance) at all times and tThe All-Ett is more than sufficient for my needs. Some people have complaints about the material feeling cheap. Frankly, the material used doesn't make a bit of difference to me as long as the wallet is durable, and the All-Ett certainly seems to be.
I also looked into the Slim Slimmy before making my decision, but I wasn't a fan of its design and read complaints about it not holding cards well. IIRC, it's made of leather anyway, so the nylon All-Ett would be thinner regardless.