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@polybikeuser
Fairly sure you'll find that the sprockets and spacers of your cassette are different widths, so you can't just replace a sprocket with a spacer. You can get single speed spacer kits (that include a sprocket) to fit 8 and 9 speed hubs, think I've got one knocking around somewhere, this may well be a better solution that cannabilising a pair of old cassettes. Might be a problem with the width of a 7 speed cassette free body being different to 8 and 9 speed cassette free bodies, but according to Sheldon Brown,
"Any 7-speed Shimano Hyperglide Freehub will actually work with 8 sprockets, without any modification! What you need to do is to use 8 of the sprockets from a 9-speed cassette, with the 9-speed spacers.
To make this work, you'll also need to use a 9-speed chain and shifters. Your old 7-speed derailer should work OK if it isn't too badly worn. The limit stops on the derailer will cause the useless 9th position on the shifters to be locked out, so this will work as a perfectly normal 8-speed rig."
So I'd guess you could get a 8/9 speed spacer kit to work with a 7 speed hub. Hope that helps :)