They can be pretty simple Lord Flynn- but I decided to push the boat out and built a frame from aluminium sections held together with greenhouse bolts. Our tub has a cone in it so feed doesn't go stale. I added extra weight to the trigger using lead free solder wire and put a cork on the end so the target is bigger. Height is important and the hole size is critical. At the recommended 20mm ours kept jamming up but at 21mm it is perfect. However I recon that if it was half a mm bigger it would just empty itself, so pellet consistency is important. Their suggestion of a shallow bowl underneath didn't work for us as our cock kept tipping it up, so we used a rubber car mat which works very well. They eat every last bit up before hitting the trigger again, so nothing there for the sparrows. It took ages to build the first one but I will simplify the frame for the second one and not bother with the cone in the tub, just stir the feed when I refill it. You could just suspend a tub with string under a shelter in its simplest form.