Why use stands. Why not isolate them from the desk by placing an Atacama isolation gel pad to each corner. A pack of 8 is about £12.99 on Amazon.co.uk. Can be used between speaker or bookshelf/stand and as far as I can tell they work well.
Just enter "Atacama isolation gel pad" into Google and you should find a number of sellers offering them. Worth a try in your situation and for that price and then if you decide that you really need stands you can use them to top out the stands.
In terms of short speaker stands you could look at the Apollo A2 (they start from a 20cm high model, and I thinkthat they also do 30cm & 40cm before getting the more run of the mill 50 cm and upwards). Like all decent stands they do have a relatively large foot print which is why I think that standas would be overkill on a desk and that the gel pads are a better bet.
Hope that helps?
Regards
Balders