Actually, I realize I squeezed in let without noticing. Try this...
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"on" sounds much more natural to me, and I think that's enough for me. Idiom counts in a language, too. In and on might seem technically interchangeable, but (sometimes) if it sounds wrong, it's wrong.
You can say, "I lose my temper." So like you can say "It is poor form to sleep in a meeting.", you can also say, "It is poor form to lose your temper in a meeting."
As someone else commented, you definitely can say, "I let loose my temper." So, "It is poor form to let loose your temper in a meeting." is fine.
But, can you say, "I loose my temper."? I think you can, but it is very very unnatural.
You can simply say: "He let loose his temper." so...
It is poor form to sleep in a meeting. It is poor form to scream in a meeting. It is poor form to loose your temper in a meeting.