Any ideas about using some sort of liquid plastic........

I want to fill this area that use to be the mounting area for a Wilderness system seat back. If I can fill the area and make it a flat surface I could use the area for a small box. Or mount a nav light pole.

Maybe epoxy putty. With a plastic welder you might be able to melt HDPE plastic to fill the area.

Marine sealant. You can buy it at home depot for about $6.00. It can be removed with some difficulty, but it won't damage your boat. You never know you, may want to change your kayak  layout the future.