I'm sorry to tell you this, but you won't find a replacement sliding lid for the Zilla Critter cages, because they aren't available. I tried to find one a while back. I even contacted Zilla's Customer Service directly and they told me that they don't even sell replacements for their own products...
Your only options are to use the rig you have or get another tank. I guess Zilla figures they make more money that way...