You could just poke the prongs with a screwdriver whenever you want to turn on the system.
Personaly, whenever I need to build a computer into something other than a computer case, I just scavenge a button off of on old computer case, they're usually just hot-glued in.
I'd suggest getting a cheapo case and tearing it down to use for parts. You could also use the motherboard tray with the expansion slots and stuff in the box and then you wouldn't have to worry about grounding issues either. Then again, I hope you're not just simply screwing the hardware onto strait wood. Are you?
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http://www.radioshack.com/search/index.jsp?kwCatId=&kw=switches&origkw=Switches&sr=1
Personaly, whenever I need to build a computer into something other than a computer case, I just scavenge a button off of on old computer case, they're usually just hot-glued in.
I'm from the UK
I wouldn't take switches from an old case. They suck.
Find good ones at an electronics supply house.
http://www.frozencpu.com/products/5814/ele-247/Military_Style_Switch_w_Yellow_Acrylic_Cover.html