Hi Erika and welcome.
I have many clients in my salon with your same problem. the diffused thinning on top and thin hair on sides and back. This type of hair typically is not strong enough to withstand a hair extension process so it is usually recommended to go with a full cap.....so you are on the right track.
for most of my full cappers, I order french lace with a sfs front, (but a french lace front works well also), I almost always orders just a little poly on the back and up to the back of the ear. This helps with keeping it bonded on strongly.
you can order a full cap by measurements or by making a template. remember that if you have very dark hair and want the front to be totally undetectable, a great tip is to order the front 1" with blond hair and color it yourself to achieve true colorless knots.
please don't hesitate to ask questions. JRob wears a full cap and is our "resident fullcap guru" LOL, his video, even though you are a woman, would still be very helpful to you.
DebbieS.