My HLT is a 30 litre URN, that has a lid that I keep on it. Helps maintain the temp and gets it up to temp quicker.
Sounds like your HLT will be like my boiler.
For that I use a large enamel tray (like a big drinks tray) for my coverted keg boiler.
But I only need a lid until it comes to the boil, then no lid required (otherwise big boilover).
Beers,
Doc