I saw one source on eBay that wants $37.99 for a set of 6 slot covers that are 15 7/8" long, total 95.25".
My slots are 5/8" wide x 1" deep. So as simple as they are I started looking for the material to make my own and came across this on Amazon;
6063 Aluminum U-Channel, Unpolished (Mill) Finish
It comes in lengths of 72" and is $21.68, Prime Free 2 Day Shipping.
So if you want to customize your slot cover lengths etc... it is fairly inexpensive.