the only construction rules out there are in the micro national rule book which is available at the following link
https://www.brca.org/documents/173-micro-handbook.html
There is no requirement for a hard case lipo, as when the rules were made, they were hard to come by. having said that, the popular softcase battery that we all use is now hard to get, so a lot of guys are now going for the hard case battery as it sits in the car a lot neater.
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Tom
TLR 22-4 2.0, TLR 22 5.0 AC Elite,
Kindly Supported by Trackside Spares, TLR, Spektrum and Horizon Hobby