seat overs for 1972 911T

[47silver]

bought some seat covers for autobahn interiors, they are vinyl comfort seats and they are well made, however...but how do you keep the center of the seat down? i looked at lakewell and they have some type of thread/twine going across the seat that is in a fabric casing. The autobohn seat covers do not have them. I called tony at autobahn and he said that they need sewn with a curved needle but where do we sew it?
I was thinking across seat like lakewells but i really do no tknow.

if anyone can show what they did or how it was done i would appreciate it.