I use sections of antifatigue mats in two layers, the interlocking kind. I used two sided carpet tape to hole the layers together and have the seams overlap to reduce the flex. Works great, can still stand and holds together even in class II+ rapids.
Sorry for the late reply, haven't been on here in a while. This is my setup, handles rivers great (up to class II+) and is a dream on the lakes. The oars are for pontoon rafts.