One possible solution to this issue is to place stoppers at the ends of the dowels to eliminate the possibility of them coming loose.
Rubber Stopper - This stopper requires no hardware and has a strong enough grip that the dowel can't be pulled out without a very large amount of force. However, it is very large relative to other options and is also the heaviest.
Rubber Clamp - This stopper gives a strong clamping force on the dowel if it is a size or two smaller than the dowel diameter (1/8" clamp to 1/4" dowel as shown). However, it does require hardware, and is relatively heavy.
Vinyl Clamp - Much like the rubber clamp although slimmer and cheaper, it gives a sufficient clamp force and does require hardware.
Rubber Band - The looped rubber band is the cheapest and lightest option, and also gives a grip force very similar to the rubber stopper.
Nylon Clamp - While lighter and cheaper than its metal counterparts, the nylon clamp provides less grip force and requires hardware.
Which of these options would you prefer?
Which of these options would you trust to support your camera? (Which would you not trust?)
Are there any other methods you have seen or used that might be more effective than these?