I think you must have a bigger LowePro than mine (TLZ2) as that wouldn't quite fit a 225mm long lens as the 100-300 is; however if you do not already carry as I do, you could try it. Lowepro strap over right shoulder & bag on left hip; camera strap round back of neck and adjusted so that it can go into bag for protection, but weight just on its own strap.

Another route losing a bit of weight and length & where I am heading to is the Sigma 50-150 with matched 2x teleconverter bringing it back to 100-300 but with about 30mm less length and 400 grs less weight (check exact http://www.sigma-imaging-uk.com/acce...-converter.htm etc). Sigma UK are very obliging types and could well do you a very good px deal (quietly).

If you don't really need 300mm the 1.4x converter is lighter and shorter. Virtually all my stuff is taken with the 50-150 on a 350D. I dare say you are as hooked as I am on a lens where all the working bits are within the barrel and flapping in and out at the front.