I recommend going to the hardware store with your crutch or at least the inner diameter of it and buy a rubber stopper that fits into the bottom hole but - with at least 1/4" hang out.  
I cannot tell you how many FS' I have seen ruined ( wet balsa floor ) due to the SS or aluminum crutch having drilled a hole through its mounting pad and into the floor - allowing rainwater or other to get into the core area - it takes a very short period of time to allow a good portion of the floor to get wet - and then, you're in for a world of hurt !  My boat is at a mooring and the constant movement of rig with the downward pressure of the boom against the boom crutch equals a drilling motion.
A .30 rubber stopper prevents this.  And, every other year I have to replace this rubber stopper due to the beating it takes.
 
 
 
 

Nice work around. However, a small warning:
At Lake Norman Community Sailing on Lake Norman, we have 14 flying Scots. Our sailors, who are mostly new to sailing, are very good at bending boom crutches. I decided to replace them (the crutches, not the sailors) with galvanized pipe material from Home Depot. The solution has worked well, except it is very easy for a novice user to tear up the boat if the boom crotch is not correctly placed into the cup on the floor of the boat...which means a fiberglass repair in an area where balsa ain't far away. I think I might try your aluminum idea, which will bend/break before tearing up boat I suspect.