Here is the final version that I am putting together as the first base. It has 2 washers, 3 layers of pennies and 3 mdf bases - a 60mm square at the bottom, a 30x40 in the middle and a 40mm across flats at the top. The mdf bases give a real tight grip on the stand. I drilled 8mm holes in them and then forced the stand through; a tit fit, whereas the washers, with a 9mm hole in the centre were loose and shaky!
I grabbed this photo on my mobile phone this morning as I rushed out the door. It shows the stand, having glued overnight, with a toy plane from my local sainsbury's (for 74p) on top. I'm pleased so far!