I thought the plastic lids were made of 1/2" PVC. If you Google you can find lots of suppliers for this. McMaster-Carr
for one. I know it is available on Amazon as well.
My thoughts on the plastic tanks is that you would loose a lot of volume and incur a lot of work with fittings and fitting in general. Just cut a new PVC lid, caulk it and screw it down and you're done.