Hi,
Alice packs are love or hate affairs...
I'd take a look first at the Tactical Tailor Website as they have a heap of mods for the
'Alice ruck'
http://www.tacticaltailor.com/ruckmods.aspx
When I had mine I looked there for inspiration.
I did the following to my
Large Alice Pack to make it a bit better. I've no experience with the Medium sized one.
1. Fastex clips on the 3 outer pockets and the lid straps.
2. I made a snow lock for the rucksack lid.
3. I added a waist belt with a Fastex buckle.
4. I added a Fastex buckle to the 'radio pocket' inside the bergen
5. Bent the frame well away from my back
6. chopped off the metal loops on the top of the frame that act as strap keepers this allowed the shoulder straps to sit better on my shoulders with weight in the Bergen.
A really good set of waterproof liners is a really good idea as the pack has the water repellant properties of tissue paper it does however float really nicely..
Here it is after a bit of work
I did read somewhere that you can ditch the metal frame on the large Alice and replace with a plastic frame that Coleman make or made..
I do know that the NZ army have an improved set of straps and waistbelt for the Alice
But only in DPM.. I've also seen them tailored with a set of Zips down the side to accept a PLCE bergan sidepouch..Which ups the carrying capacity to something horrendous for a pack with no proper waist belt...
They are generally horrendous to carry with any weight in IMHO and after 24 or so months of using it I was totally ready to move onto a different Bergen. It reminded me of lugging the old issue SAS / Para Bergen around..

not a pleasant experience either...
Hope this helps