If you do use a small ball float, you'll need to insert a stainless washer between the elbow (or whatever fitting you use) going into the kettle and the sight gauge so make the opening a bit smaller, otherwise that ball will get sucked into the kettle when the water gets lower. Just pull the glass (plastic) tube out of the fitting and drop a stainless washer in it then place the tube back in.
I had a bit of a hard time seeing my water level when the background is stainless steel from the kettle. What I did was I fed out a long length of white P-Touch label (just kept pushing the feed button until I got the length I need) and stuck it to the back of the sight glass to give me a white background. Is it perfect? No, does it help, to me it helped a bit and best of all, didn't cost a thing since I already had a P-Touch.