I'm not confused... but my post may have been confusing.
Three battery mounting choices:
1 - saddle pack -> cut out edges on lower chassis
2 - stick pack -> use plastic battery tray
3 - side by side -> use plastic battery tray BUT
you can't get a side by side into the battery tray because they're too wide at the battery bars & wire solder points. You'll need to widen those areas to use the plastic battery tray.
Your method with velcro is probably the easiest if you have other cars that use side by side packs.