I use an app called SlowShutterCam that allows 1/4 1/2 1s 2s 4s 8s 15s 30s and 60 second exposure times regardless of how bright it is. Essential it shoots video and overlays all the frames to create one image. I bought a
Griptight Mount tripod adapter by a firm called Joby that works really well with various mobile phones and seems to hold the iPhone in its case very solid. I recently bought a lightweight Manfrotto tripod (see below) specifically to take out with the dog on our walks as even though my main tripod is a carbon fibre Manfrotto its still an effort to lug it about.