Yesterday I came up with a solution that cost nothing (0) and works very well.
I got a piece of approx. 1 inch by 2 inch (25mm x 50mm) pine (although hardwood would be stronger), just a few cms longer than the distance from the ground to the middle of the rear axle. Then I marked the position of the middle of the axle on it with it positioned vertically next to the right side of the rear wheel. About 20 mm above this, I drilled a hole right through the timber, very slightly smaller in diameter than a 150mm (6 inch) flat-head nail, and banged the nail right through it.
Here's how it works: with the bike on its side-stand, I stick the nail into the rear axle, then tip the bike over towards the left. (The easiest way is to stand beside it, then grab the subframe below the righthand side of the seat, and pull/lift it towards you so it leans against your right leg). When you do this, the piece of wood drops down vertically, so when you stop leaning the bike to the left, the wood is in the correct position, directly below the axle. The bike is now supported by the sidestand and the new Heath Robinson axle support.

If you're not sure what I'm talking about, I'll take some pics and post them later.