On my IP-056 I just have the seat clamp that Peter supplied at no cost with the frame.
On my IP-036 I splurged and went with a Thomson seat collar, it's a wider design just like the Loaded brand clamp. It's a bit more expensive at $25-$30 but it is made by a company not far from me in Georgia.
Both work well and have not given me any issues, I do use carbon paste (Finishline Fiber Grip) but even before I started using that product, I wasn't having any slippage issues.