If you can't get golem buff reliably, there is always Glacial Shroud... people severely underrate this great situational item for casters. It is quite cheap, gives loads of mana, a huge 20% cd reduction (this is enough to get close to 40% already with just runes and masteries, no golem buff needed), and the 45 armor is never really wasted.
It will make a big difference vs any incidental damage you take from minions or towers and be a pretty big help vs most champs even if they aren't physical, since you will also be much harder to harass with any ranged auto-attacks or physical-based aoes, even Tibbers will hurt quite a bit less, since his attacks at least are physical. It also has some synergy with archangel's staff, which a lot of casters like to get anyway, making it even more economical.
The next time you find yourself up against a hard aggressive physical champ and have to buy a chain mail, think about upgrading it into this. Short CDs can make a huge difference and you will definitely feel it. If you have any kind of cc abilities it WILL end up saving your ass a lot, and it can be upgraded later to frozen heart if necessary for more defense and even more utility with a great aura. That's a lot of good stuff from one item for just the cost of 'wasting' 700 gold on a little armor.