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Using a whole cassette is a good shout. Maybe the next step up would be to use just use three of the cassette sprockets. Chain would sit on the middle one and be blocked on both sides.
I'm thinking that a more permanent alternative might be to find a NOS red Mavic Open Pro rim and build a dedicated SS rear wheel. The more I think about this the more I like the idea of having a SS permanently.
If it’s only temporary and for minimum work just remove the derailleur, shorten the chain and keep the cassette on. If the chain tension is right and in line it would be fine to use and easy to put back to geared when you need to.