OK here is the story. I have a window box type planter on my deck and it is not doing well because
the coconut fiber base is falling apart and now the plants are about to fall through. On one end the
soil has already fallen through. It is a metalic planter which had a coconut fiber base and I am wondering if I should replace it with more coconut fiber or should I use landscape fabric.
The dimensions of the window box type planter are 24 inches long x 6 inches deep by 4 inches wide. The bottom is a lengthwise and it is a long metal band or pole.

I'm not sure I'm picturing what you have. Do you mean it is a sort of cradle made of metal rods that support a coconut liner of the same shape? Those are pretty common.

I'd go back with another coco fiber liner,. Amazon has several sizes. But you can improve it by lining the inside of the liner with plastic with a few holes punched in the bottom. And if you have some garden cloth around, that works, too. That will keep it from drying out so quickly and should also help preserve the integrity of the liner. And when it does break down, the plastic of garden cloth will keep the soil in place.

Well I did some more coco liner and replanted the Asparagus macowanii 'Tufted Asparagus Fern -- Ming'
I may try to figure out some other plants to put in the planter later.