I have used a small pill bottle set in the plywood tip rib of a Twister. I have also taken wing mount blocks, tapped 1/4 x 20, and dished them out some inside, then build a ply box around them and the balsa around them for the silk to stick to. I use steel BBs put in through the bolt hole, then close the hole with a cut off piece of 1/4 x 20 wing mount bolt.