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Toy Splitter Ax From CardboardView Instructable »
Collector for Tabletennis BallsView Instructable »
The screw of the cap could be used to switch power on/off . To avoid the cap to fall when not tightened (power off) you can use a pin fixed on the bottle screw and a corresponding buttonhole on the cap that allows the required angle from on to off position.
Thank you :-) Battery replacement could be difficult if the plastic caps are rigid and the tie wraps are very tightened. If you need to change battery very often you can try to use 3 tie wraps at 120 degrees instead of 4 and adjust their tension to keep the battery in place and allow replacement at the same time. Since I use rechargeable batteries I prefer the robustness of the connection.