I've made several pairs of solid aluminum crank arms for my Hobie Adventure. My purpose was to experiment with different crank lengths, not to deal with breakage, since I'e never broken one. If you want more details, see this thread on the NCKA forum:
http://www.norcalkayakanglers.com/index ... ic=54481.0The first pair I made had some issues, but the rest all turned out very nice. Btw, if you want the precise measurements I used (drill bit sizes, spacing, etc.), go to post #15 in the NCKA thread and click on "dimensions.pdf"---it appears just above the pictures (look closely, as it's easy to miss).
All of the cranks I've made so far used solid 6061-T6 aluminum bar. Locally, I can get the metal for a pair of cranks for about $10, and it's less than that online. I'd be willing to bet that you couldn't damage them if you tried.
The last I heard, the guy who is selling crank arms on the NWKA forum (he also sells them on the NCKA forum that I frequent) is charging $125 per pair. According to discussions on the forum, those cranks are made out of solid 7075 aluminum, they're anodized, they're drilled for 9/16" bike pedals, and they include a nifty stainless spindle that allows you to use the standard Hobie pedals. I'm not saying they aren't worth the money, but IMHO you can make your own bulletproof crank arms for a small fraction of that price.
DIY only makes sense if you are OK with bike pedals (as opposed to the standard Hobie flat "pedals") and provided that you have (or can borrow) a drill press. If so, the only significant expense is the taps needed for cutting bike pedal threads. Such taps can be had for about $40 a pair (less for the 1/2" pedals that I use). But, if you only need one or two sets of cranks, there's no point in buying the taps, since any respectable bike shop should be able to cut the threads for a small fee.
Just something you might want to consider...