Blu-tack is very effective and offers the added advantage of not damaging the bottom of the speakers.
To experiment with an inexpensive alternative, you might consider Topstick Hairpiece Tape.
Topstick is widely used in TV/Film production to secure all sorts of items besides wigs. It's removable, and in my experience leaves no residue that can't easily be cleaned off with some mild detergent.
I disagree with Wendell in that while it is a very effective coupler it changes the sound of the speakers I use, which are both ported and closed box. If your top end is a little bright it will bring it down considerably. But many, including Martin Colloms agree with Wendell. My own experiences have led me to prefer cones or my current preference, roller bearings. Not for the faint hearted or those with small children or large pets. There are real differences between the sounds of these that stem from whether the speaker is coupled or isolated from the stand and what the vibrational characteristics of the stand is. But the Blu-Tak or its generic versions is the cheaper way to go so try it first.
Herbies Big Fat Black Dots are an excellent interface material. I use them between the lower and upper modules of my VSA VR4JR speakers with a significant improvement in clarity and dynamics and use them also for my monitors on stands. They come with a money back guarantee But I'd guess you won't part with them once you give them a try.
A very inexpensive alternative is to use V-Pads available from heating and air conditioning suppliers for about $3 @and are identical to Mapleshade IsoBlocks.
I found each of these approaches vastly superior to blue tack.
The M20s have threaded holes in the bottom. When I owned the M20s I mounted them to my Sound Anchor stands via a plate I made with holes drilled to match the M20 mounting holes. I had to drill counter sunk holes in the plates to match the stands mounting holes. Once complete the speakers were firmly coupled.
The next question is, was the sound improved?
There are so many options and so many opinions that you really have to try a few possibilities for yourself: Herbie's Big Fat Dots, DIY Sorbothane discs (from material purchased through McMaster.com), Nimbus couplers, Grand Prix Audio Apex footers, Aurios, ArtGum Erasers...the list gets pretty long.
BTW, I thought Sound Anchor supplied coupling discs of some sort? Maybe I heard this wrong or I'm thinking of another manufacturer.