Hmm. Here's an idea that can be adapted with a bit of ingenuity.
Get hold of two plastic discs around 25cm in diameter (Wood discs would also do as long as they were treated with an environmentally friendly preservative and left to dry). Cut a hole in the centre of one disc to take a piece of fine mesh or a fine mesh bowl (made yourself if need be). The size should be about 10cm in diameter. the idea is that the bowl or mesh is saucer shaped and fits snugly in the centre of the disc.
The next step is to buy a bird feeder stand. They're freely available in garden centres, DIY stores etc for around a tenner. They basically consist of a two section metal pole that has hooks to take birdfeeders at the top. They also contain a mesh bowl of the size you're going to need.
Mount the mesh bearing disc or bowl on the centre of the pole by driling a hole through the mesh. It must be a tight fit on the pole. Slide the disc down the pole to a suitable height. Then take the other disc and drill a hole in the centre to allow you to push the disc down on the pole. Again, it should be a fairly tight fit.
Position the top disc about three inches above the bottom disc. This way you'll find that the device will allow the small birds to get in but exclude large birds like crows or wood pigeons. If you want to feed larger birds, just place the construction somewhere nearby.