Quote Originally Posted by aglondier View Post
Sounds like you have a serious case of Hostile-DM-itis...
many people seem to have that reaction as soon as they hear of targeting items, without further informations. it's not necessarily the case. it can be a competitive game, or it can be an inexperienced dm. I always had enemies spam disjunctions in high level fight, but I also provided more than enough replacements (though I never included bags of holding among targets, for reasons of practicality and bookkeeping).
in this case, I really think it's not hostile DM. an hostile dm would just sunder their items and laugh at the players' lack of foresight. the fact that he gives forewarning means he wants you to adapt, which means he wants to challenge you tactically.

Quote Originally Posted by Fizban View Post
Never use magic items made of anything less than metal, but aside from that, stay out of range, be in cover, miss chances ought to carry over. etc.

To protect the sack, stop wearing it and put it in another sack. Now it has full cover and cannot be targeted, or destroyed by natural 1s on saving throws. The outer bag can, but attacking the outer bag does nothing to the inner bag, and depending on just how "destroyed" it is by the attack, you can claim some amount of cover for the inner bag. Or just put the sack inside of another sack, etc. Use sacks made of chainmail and they get that 10 hardness.
i have seen that claimed many times as a way of protecting items, but it never makes any sense. Let's substitute the item to be protected with a person, and see how well it holds
"to protect yourself, put yourself in a sack. Now you have full cover and cannot be targeted. your clothing can, but attacking your clothes does nothing to you"
we may as well go with
"throw a towel on your head to protect yourself from the bugblatter dm of traal. the bugblatter dm of traal assumes that if there is something blocking direct sight between you and your foes, you cannot be seen, targeted or attacked."
I have never seen anyone claim that putting on a burqua makes you untargetable and invulnerable, yet people make the same argument for items. this seems hostile player-ing to me: here I am offering you an in-world puzzle, one for which I forewarned you in advance, and instead of engaging it within the framework of the game, you try to rules-layer your way around it.

besides, the point of the handy haversack is to be able to retrieve items from it quickly. if you have to open multiple sacks to get it, retrieving items is no longer a move action.

that said, covering it in chainmail would absolutely provide a modicum of protection. discuss the option with your dm; if he gave warning about targeting items, he likely is not adversarial, and he wants you to engage the challenge smartly. an attempt to tackle the challenge within a realistic in-world framework should be well received.

protecting/hiding the item under the armor can be done for an amulet or similar, but the haversack is a backpack, you can't fit it under your clothes. putting the haversack inside another nonmagical backpack could be a functional way of disguising it; if you are a trained adventurer, you may be able to recognize specific items; and in that case, you should recognize that the other guy is wearing a nonmagical backpack, not worth sundering. but that depends on what your foes want, and how much intelligence they have on you; it's an "ask your dm" situation.
In general, the haversack seem very vulnerable, and I can't see ways to protect it very well. so I'd go with counterattack and mitigation. how much stuff can you really have inside anyway? remove the most expensive things from the haversack, keep only basic potions in it. this way, if the enemies manage to sunder it, you don't lose much. try to kill them before they can sunder your items. the loot on the enemies is likely to far exceed what you may lose from the sundering.