The only suitable 'plastic' container you are going to find for ANY monitor over 8" is going to a Rubbermaid Cattle Tank. If you have a farm/feed supply shop near you, I would look into it. The 5 ft long model should be good until the monitor gets about 2 ft. long or so. For an minimum adult enclosure I would go to the 300 gallon model and build a top for it. They also sell galvanized stocktanks that do great and get up to 10' diameter circles too.
I use a 150 gallon(like 58" x 39" x 25" tall) Rubbermaid stock for my Blackheaded monitors....the only reason that works is because Blackheads are not really borrowers..
....screen tops and multiple lights are only good for making a big batch of beef jerky.
....and it will crack under the weight of the 1'+ of substrate that a sav needs for borrowing.