How can I stop a toilet seat from moving?

We now have a lovely modern toilet (extra high) with the square, soft closing toilet seat. This can be dismantled to clean if wished. Our problem is it keeps coming loose & the only way my husband can tighten it is to lie on his back (he is predominantly left handed) & struggles to do it only for it to become very loose again, the underneath screw is set so far into the loo, if that makes any sense. We are both in out 70s & not as agile as we used to be!!! Any ideas please???

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