You can try using extra straps, or putting a folded kipmat inside the frame, but the problem is that the balance of the bergan means the bottom strap is the one that bears the most weight and consequently bites into your back.
I've seen people use the old CEFO "rhino pad" to spread the load at the base and to provide a handy waist-strap, but in my unit that invited severe piss-taking and insinuations of gender and airborne-worthiness... Acceptable solution was to fill the unmodified bergan with rocks, tab about 100 clicks, and endure several generations of weeping raw bergan burns until they formed callouses.
Or you could just use a modern rucsack from Millets.....
In which case you're better off with the complete US Alice pack (ukkitmonster will have one), maybe try and get the Kiwi version as their DPM is similar to ours.
Not sure what version the Spams are on now, but the Alice pack was replaced by the Malice pack (great imagination) and now they're getting into packs with molly straps to allow custom-fit pouches (as we will soon I am led to believe)
I never really got on with the external frame bergen but I quite like the look of the Kiwi one I've seen in use (similar to the one Regiment646 links to above)
They are still issued, the PJI's then don't say a word about yr bergan and everything in it. The most under used piece of kit but hides loads of problems and prevent in tanglements too!! Ask yr CQ for one