When we do pull sets, we almost never use pull buoys. Instead, we use a doubled-up kick band wrapped around the ankles. It's significantly harder to pull this way but we feel that it's more effective.
The other day I was reading "Swimmer and Team" by a legendary American coach Don Gambril and found an interesting image of a device that he used in his pull sets.
As you can see, it's an innertube wrapped around the ankles. Sometimes, Don used innertubes and buoys together in order to give legs greater support. I have never tried combing the two, but next time I pull, I am going to try it.