Hello Carl,
The most important aspect that you need to consider when choosing an enclosure for a trace heating system is ensuring a suitable level of protection from the surrounding environment. When installed outdoor, enclosures for thermostats used in trace heating systems need to prevent ingress of dirt and water as well as withstanding exposure to sunlight and extreme temperatures.
In addition, it is helpful to select a one-stop-shop for thermostats and enclosures. This helps streamline supply, minimise the number of contractors as well as to ensure maximum compatibility between the two components.   
To address these issues, Spelsberg offers its HT range of thermostats encased in its IP66-rated TK enclosures made of polycarbonate (PC). In this way, it is possible to provide ingress protection against dust and water, resistance to UV and a wide range of tolerated temperatures, spanning from -35°C to 80°C.
HT thermostats and TK PC enclosures are combined to ensure seamless integration and delivered readily assembled, minimising time and cost associated with installation. In addition, Spelsberg is able to customise and supply specialist solutions that meet the specific requirements of your trace heating system, if your requirements are not satisfied by any of the TK PC enclosures available.
To learn more about Spelsberg’s HT range of thermostats, visit: https://www.spelsberg.co.uk/news/news-press/detail/specifying-enclosures-for-environmental-management-systems-in-a-smart-world/
For additional information about our TK series of enclosures and to look at the product range, visit: https://www.spelsberg.co.uk/industrial-housing/with-/-without-metric-knock-outs/
If you have additional questions or would like a bespoke enclosure, please feel free to send me a private message to discuss the requirements that your enclosure needs to meet.