I have an older bike with a 7 speed Shimano 105 groupset that is in need of a new rear wheel. Based on the following from Sheldon Brown, I figured I’d be fine with a 8/9/10 130mm hub and 4.5mm spacer:
Also, hubs marked "8-speed", "9-speed" or "10-speed" will work with any number of sprockets! (Add a 4.5 mm spacer before installing a 7-speed cassette on an 8-, 9-, or 10-speed hub., and the included 1-mm spacer before installing a 10-speed cassettes on an 8- or 9- speed hub.)

So I bought a new rear wheel with a 8/9/10 speed, 130mm Shimano 105 hub. I also bought a 4.5mm spacer.

Now I’m trying to install the old cassette on the new hub using the 4.5mm spacer and having problems. First I drop the 4.5mm spacer on the hub. Next I install the cluster with the six cogs followed by the spacer (not the 4.5mm spacer but the spacer that is part of the cassette) and finally the seventh cog. The problem is that while the first six cogs and the spacer are held in place by the hub’s splines, the seventh cog sits just outside the splines and rotates freely.

Not sure if I’m doing something wrong or if I got the wrong spacer. I’m guessing I’m doing something wrong but I found this thread in which someone else reports that a 4.5mm spacer was too big (see post #17) which makes me wonder. Any thoughts?

Also, I’ve noticed that if I put the 4.5mm spacer on the hub between the cassette and the lockring everything fits and I don’t run into the problem of the 7th cog spinning freely. I am however hesitant to try this as Sheldon says to install the 4.5mm spacer before. Any thoughts on the suitability of this fix?