If it's new never used you need to run it at full power for a specific amount of time not in your enclosure to burn off any residual oils and chemicals from the manufacturing process before putting in your enclosure. As long as it fits inside you should be fine. You just won't be able to create an ambient temperature gradient. So the whole enclosure will be a set ambient temperature and your hopefully thermostat controlled UTH will create the hot spot so you will be fine. Make sure to have the UTH set up correctly and I see no worries.

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