Mine are covered with 1/2" grey PVC. I think Plexi is too brittle. When I installed sending units that are the metal strips in pvc pipe, I just drilled a hole in the top of the tank and tapped it and the sending unit screwed right in. They also have the access plates glued into them.

http://www.usplastic.com/catalog/item.aspx?itemid=29585&catid=733