Just got a new sail and realized that it requires a 187 boom... while mine is max 185!!!
How critical these 2 cm are?... wouldn't like to buy another boom! 
I tried it once, and it really didn't work 
Sail was way under outhauled, felt like a sack of potatoes 
If you were stranded on a desert island, with only that equipment available, and you needed to escape into the wild blue yonder and rescue yourself, then...
Under downhaul the sail. This will mean you'll need less boom length, so you might just be able to get enough outhaul on the sail to make it manageable. Perhaps. Enough to escape the island, anyway
In the old days, there were booms around that extended by adding extra bits of tubing. If you can find some bits from one of these you may be able to improvise. (some surfshops have a bin full of old odds and ends)
But be warned!!! you'll be putting more strain on the boom than it was designed for.
If you're only light and it's a smallish sail you may get away with it.
what size is the sail? Depending on wind, I may have my sail rigged underhauled, if it's light. Also depends on where you sail and your standard of sailing.
