Backpacks for longer trips is a difficult subject. I have had a number of different kinds over the years, but never found something I have been really happy with.
Currently I have a Lowepro protrekker 400 AW for the more serious trips, which includes my 400mm f2.8L II, tripod etc. But when I go on a multi day trip in the mountains, I need a tent, sleeping bag, food, clothing etc. Then the protrekker is insufficient and the solution has been a more regular large pack, like a Bergans Alpinist (130 liters), with a fairly unsatisfactory packing of the photo gear in between all the rest. I once tried to make a padded photo section in the bottom part of the pack, but it didnīt really work. I have not tried the TrekPak inserts. If you get it and try, Iīd be interested in learning about your experiences.
For holiday trips etc. I use a Tamrac 5549 Adventure 9. That is a much smaller, but quite a convenient pack for a (up to 1DX) body, a selection of lenses (I normally carry a 16-35mm f2.8L II, a 24-70mm f2.8L II and the 70-200 f2.8L II. I sometimes also squeeze in a fast prime, 50mm 1.2L or 85mm 1.2L, or the 100mm f2.8L Macro IS. There is an extra room for items like a flash, an extender, a sweater or a book, a good room for a laptop and a place for a bottle. Itīs also easy to carry on any plane or where ever you go on a regular vacation. I have actually had that pack longer than any of the others.
How did you plan to use the insert (to fill a bag or to use it as a part of the total package)?