You will probably need to install hip pads. Glue the pads on either side of the seat using two layers of contact cement. Cover the foam with a thin layer of neoprene for additional comfort. This is also practical if you wear a thick wet suit in cold seasons and change to shorts in the summer. Hip pads that are too tight will cut off circulation to your lower limbs.
Most recreational kayaks come with molded thigh braces.
The plastic can chafe your knees, so I recommend cementing a thin layer of minicell foam or neoprene over the entire surface. This also keeps your knees from sliding around. Some kayak designs have different thigh-brace inserts. You might try these for the most comfortable and supportive fit. Foot Rests - Feet and legs play a major part in kayak technique and propulsion.