I had a similar issue with my Predator when I had it and I did as you've already thought and velcroed it to the inside of the shell. It made it easy to swap between cars and saved squeezing it in somewhere and trying to shoe horn it in and out evertime you wanted to swap it over.
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Losi Eight T - Nova Rossi P21-7T- Hitec 5955's & Spektrum + TeknoRC Losi 8ET - Neu 1515-1Y - MGM 16025 & Hitec 5995's.
RC Heli's - Align T Rex 600 Nitro Sport - X-Cell Spectra G gasser.