Arkel thought of everything when they made this trunk bag.
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Topmount trunks fasten to the top of a rack.
By definition, this means you have to have a rack. You also have the weight up high.
These packing devices are usually quite convenient, but carry a price that's measure in weight. They can also reduce the useability of the rack for panniers, since they often occupy the top rails of the rack and prevent other bags from mounting there.