Heaters do okay in plastic - I have had two of mine in kritter keepers with 30w adjustable mini heaters and they're fine.
But putting a heater in that -shape- is really difficult... For twelve dollars, you could get a Kritter Keeper or the like with the same gallonage or more, with flat sides that will support a heater's suction cups way more easily.
The XL is 5.90 gallons, apparently, and the L is 3 gallons.
Though I notice a few people say on the reviews there that the tank arrived in the post cracked.. so maybe worthwhile buying from a pet store rather than by post.
The only heaters I've seen that will sit a bowl shape well were not terribly good or reliable, little heat pad things.