You can also use the rules for supernatural item building from the Companion.
Single-use items fall apart after being used, like grenades. Multi-use use could be the most common ones, since they eventualy run out of "fuel" and then need to be recharged. Permanent supernatural items are fueled by perpetual motion engines or such.
In my oppinion, powers themselves would represent innate capabilities of the individual -- like psi. If the effects themselves originate from the gadget itself -- telekinesis raygun insted of a helm that enables telekinesis through mindwaves.
If characters themselves do not have access to powers, you might like the Artificer Training feat from the Adept's Handbook -- if adepts are not available, make it a expert feat then. Just change its name to "crypto-engineering" or "mad science" and you're ready to go.