Well, the backrest will be more comfortable, no doubt... I'd just caution passengers not to lean back on it. Ideally, they should either be gripping you, or the rack, and ready to lean side-to-side, as you do, in turns... and NOT leaning back against the topcase. I don't think you can make that plastic secure enough to safely support an adult leaning back on it. It may not fail immediately, but it probably will, at some point. That's too risky for my tastes.
I'm sure you could find a "sissy bar" seat or rack attachment that would be all metal, and more capable of supporting a passenger's leaning against it, if that's what you need, though.
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