Is the screw long enough? It shouldn't fall out at all! But I will say this. I was at a str party last month, and I saw someone with a Cassigrain scope and he had a lanyard attached to his diagonal prism piece. Interesting. Perhaps your problem is not so uncommon. Try using thin teflon tape around the barrel if you can. The potential risk of exspensive optics hitting the ground would give me ulcers!