I can entirely sympathise with you. It's awful when they disappear and even a few minutes can feel like a lifetime. I have some issues with Penny as well, as she has a really high prey drive, and while her running off chasing birds etc is not quite so bad where I can see or hear her, we have really thick pine forest on some of our walks and I just can't take the risk. I try to make sure at least one of our walks is in a safe place that I can let her off freely, so that if we walk through forestry land, I keep her on the extending lead. I've not found a long line that useful in thick cover because it gets tangled up, but the extending lead is a fair compromise.
I don't know whether it would work with Louis, but I keep Penny quite hungry in the day time. It has made her much more food orientated, where she used to be indifferent to treats unless they were really high value such as sausage. It also stopped her leaving half or all of her meals as she used to do. I cut down the amount of food I was giving her over a few days, even though she's very slim and it went against all my instincts, and eventually it got to the point where she woofed down her meals and din't leave anything. Now she's in the habit of eating properly and is maintaining her weight again, and will pretty much work for any titbit, even kibble.