I soldered a powercable to my powerbank to make the whole system more robust for long-distance walking. Fortunately, I discovered just in time that I would never have got that thing past airport security! So I packed a conventional one instead.
I have modified an electric travel shaver to make it plug into a solarpanel or powerbank instead of using two replaceable AA-size batteries. Not only does this save weight, but it prevents it from failing. My experience is that humid conditions corrode the battery contacts and make the shaver fail after some time.
My Terra Nova Superlite tent was on my request modified by the manufacturer to make it more windproof. They added two guy-lines at the front and an extra hook to fix the flysheet to the inner tent. This was not my own idea, I got it from the internet somewhere. It's a much better tent now so I don't know why the manufacturer doesn't make this a standard feature.
Made own pot-cozy, saving lost of gas.
I modified one of the side pockets of my Osprey Aether 70 backpack, glueing in a piece of plastic to prevent scratching of the screen of my phone that I carry in that pocket (used as a navigation device and field guide).