Could you mount the light under the front bag, i.e. at the height of the fork crown? You wouldn't get any stray light in your eyes as the bag would hide the light from you but with it being lower on the bike then the shadows it casts are going to be longer though the Exposure on your head should fill those in to some extent.
Unless the bar extender brings the light forward and high enough then the bag is going to block a lot of the trail.
Better weight than wisdom, a traveller cannot carry
I was going to make one, but after seeing the other thread about the same problem I just bought the Bar Fly one which works really well and is probably lighter than I could make one. The fitted harness is a 'mini' version I made to hold a small dry bag for my waterproof. With this and a saddle bag for pump/tube I now don't need a rucksack on an evening ride.
how ever ive got a maverick sc32 at the front and it doesnt provide many options to fit a light down there bar clamping it onto the fork leg - which puts the lights the high risk zone....
I tend to adjust the power output to suit the conditions to maximise battery life, full power for descents and low for climbs, so I needed the switch on the back to be handy. I also point the light down on road sections to avoid blinding oncoming traffic, which is easy as the light is only held on with a band. I did make an adaptor to fit the light to a strap, it was a piece of plastic pipe slotted 3/4 of the way along it's length which slid onto the strap and the band went round that, but it tended to wobble.
I had a Solarstorm X2 and a Garmin Oregon on a Minoura Space Grip bar extender, worked fine last weekend. I had plenty of light over the top of my bars
Some of the bigger bouts of rain seemed to have upset the light though and there's some condensation inside, but that's a whole different problem
[I also might have just splurged on a couple of new bike lights)