Incredibly easy, but I couldn't find anyone else who had published this with a quick search, so I thought I'd post it quickly.
Had to remove both of the top breadboard power strips because they were interlocked. Then replaced just one with mounting tape and screws. Hole for other power strip visible just above (+) on power strip.
Used punch to mark where mounting holes are on Arduino. Drilled 1/8" holes with cordless hand drill (put wood behind breadboard for when drill punched through). Cleaned up edges with round file. Mounted with short 4-40 bolts/nuts (nuts just hand tightened - I didn't want to warp Arduino).