The memory of function spell can restore a destroyed construct provided that you have all the pieces. It's 10,000 gp per cast, but that's still better than crafting a new one.
Apart from that, keep it out if harm's way, same as if you had a magic item too powerful to repair. On the bright side, as a golem it's much easier to heal than most constructs and immune or resistant to many common damaging effects.
You could also add more HD or the advanced simple template to it:
Note: The market price of a golem with more Hit Dice than the typical golem described in each entry is increased by 5,000 gp for each additional Hit Die it possesses beyond the standard for its kind, and increases by an additional 50,000 gp if the golem's size increases. Building a golem with the advanced simple template increases its cost by 15,000 gp.
If money permits. Otherwise keep some make whole at the ready.
Ruyan.
I'm curious as to why you would want this thing? Sure it would impress the peasants, but CR appropriate encounters will likely ignore it or destroy it fairly soon, especially as you advance into the upper teens.
Faking a negative hp border with proper programming might help a bit. Simply make the golem perform an emergency shutdown once it gets below a certain threshold. Assuming he doesn't suffer what any of the players might as well encounter - that is getting hit so much in a single hit to go from half hp to very dead - that should save the construct from being truly destroyed, instead making it wait until you can fix it enough. Of course it has the downside of cutting his effective hp with which he would keep fighting.
mountain troll flesh golem?