I really hate it when this subject comes up.

Having already filled one diaper with the innards of my engine going 170mph, I can say without hesitation that I am all for any rule requiring an engine diaper.

As far as it holding in engine heat, I wouldn't worry about it unless your car spends time on the street as well as the track.

We have a few cars that run at our track that made a belly-pan with an absorbant mat secured into the bottom, and it was accepted by our tech department. We are IHRA, but that doesn't matter. It is required for anything running 9.99 and quicker. Personally I don't see what difference ET makes for setting the rule, slower cars use engine oil just like my car does. But that's just my opinion.