I found a personal solution to prevent my Thermarest from slipping away in the hammock during i try to sleep on it.
I sewed a "pad-holder" and attached buttons on each corner and in the middle of its length.
I figured out the way i sleep in the hammock and marked places for the connections. On both sides of the fabric of the hammock I added self-adhesive nylon-patches to strengthen the textil. There i added belt-straps as a connection to the buttons.
All the sewed connections are made with a strong yarn.
The holder is large enough to carry a thick winter-pad and i will test it during the following winter. It has already worked for me several nights.