I built a jetbike model a long time ago (way before my Eldar) and I figured out a way to make a fairly sturdy and cheap magnetic flying bases for my model.
Here's what you'll need... an old pen, your flying stand, a small screw, some greenstuff, superglue and a suitable magnet.
I grabbed an old pen out of the kitchen drawer and a screw from my workbench. You don't want too big of a screw otherwise it will damage the jetbike when you go to put the whole thing together.
As far as magnets go, there are all kinds out there from regular ones to Rare Earth. You just need something strong enough to hold up the jetbike. The one I used here is from Michaels Craft store and it fits perfectly inside the pen tube.
Assemble it like so and you're in business. They go together pretty quick and it's more than enough to hold up your average jetbike (even with some metal bits). You can paint the pen tube afterwards and leave the bottom of the flying base clear or give it the same treatment as the rest of your army.