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Unscrew the clamp bolt, insert a penny in the slot, and carefully tighten the bolt against the penny. This will increase the diameter and simplify the installation. You'll most likely have to put the bolt in backwards, from the other side. Found this tip on another bike forum, and just did it over the weekend successfuly. There was a point where I thought it would be imposible, before trying this trick.
Others suggest filing down edges of the stem's clamping area to make them more round (because the edge is the the only that catches when installing, but the middle part of the clamping area is the one that actually holds the bar in place).
Decathlon 25.4 compact drops: approved.