We always carry two LPG bottles when we travel overland. One bottle for cooking and the second for lighting.
In the early days I used to strap the bottles to the top of the roof rack using a ratchet strap. This was not all that safe, so after checking out a few gas bottle holders in the market I settled for the Front Runner universal clamp. This has worked very well for me over time. The beauty is, since it is a universal fit, if I was to upgrade the bottles then I can use the same bracket.
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Front Runner Gas Bottle Holder:
(I am not in any way associated to Front Runner or any of their dealers. I am simply sharing with the wider overlanding community a product that has worked well for me.)