This is not a DIY canister light but I was at the Houston dive show last weekend and at the Dive Rite booth they had this light:
SCUBA Diving Equipment and gear for Technical, Wreck and Cave Diving: Dive Rite, Inc - Product Catalog - LED 500 Corded Flashlight
It is a standard light that you can buy a cable attachment for and turn it into a canister light. The new LED 500 is as bright as their 10W HID. I e-mailed Dive Rite and they do sell a Goodman handle for this unit. He also slammed the light against the desk to show it's imapct resistance compared to an HID. For the cost I am thinking of going this route. It does take AA batteries which could be a good and bad thing.