Cassette speed Elite Turbo trainer 8 speed on 11 speed

Hi all, I thought that the Elite Suito was compatable with 8 speed before I bought it but it is officially only compatable with 9-10-11 speed.
I have searched everywhere on the web and (and here) and the only advice I can find is that by adding the correct amount of spacers, my 8 speed will ride smoothly.
The Suito comes with two spacers and I tried adding both and then only one, but out of my 8 speed triple, I can use around 7 gears.
I do not want to damage the cassette, chain etc, so I would like advice on how many spacers I should add to make it run smoothly and what mm size etc they should be. If anyone can help I would be relieved. Getting my hands on spacers has proven a little hard too. I tried 3 bike shops today that said they did not have any and to buy just one online comes with a 10 pound postage fee!