The rear foot post (also called extrinsic heel post) can be made out of soft material. The soft material, like birkocork shown here, will need to be replaced every 3 to 5 years, but gentler on the heel and knee/hip joints when the heel strikes the ground than the more durable plastic based posts. No top cover is better than the original Spenco (although a knockoff called neolon is shown here). Between the top cover and the plastic when there is heel pain, more Spenco or Poron can be used.
Here is an example of a plastic post, placed on a plastic orthotic, and the right side is even worse with an additional plastic heel lift. No wonder the heel kept hurting. Think soft heel and supportive arch for the best results.
Adhesive backed gel sheets can now be used, normally 1/8", in the heel area, and then covered with Spenco to protect it. This is only for walking, but sure is cushy!! With sports, gel is too unstable a surface. You can order this from the link below.
So, with plantar fasciits/heel pain, the success or failure of your conservative treatment may rest on the ability of your orthotic devices to soften the heel and transfer weight into the arch successfully. Good luck!!