hum... risk losing digits and accedents prone to metal construction and appoxy mixing as well as trying to procure such products (co2 canisters of that general size tend to be some what illegal) as well as needing a basic knowledge in crafting. as well as risk alergic reactions from paint or the shotty material that co2 canisters contain as well as poisioning from the paint or apoxy, to construct a very small metal dildo
Or, take a 10 minute drive to your local sex shop, buy a perfectly safe legit pre made dildo out of safe materials professionaly made and fitted to your own specilized feel. pre made dildo for 29.99 (the cheep ass kind)
If you want ingenuity, I could tell you about the time when I used a large-diameter piece of flexible tubing, a broom handle, some duct tape and some random parts and hooked them up to the motor on my parent's treadmill, and had a simulated blowjob/fuck. Was very involved yet simply produced, and it was amazing! When I was younger, I used to create all kinds of cool ways of masturbating.
Reciprocating saws/sawzalls are very good! To control the in and out action, plug the saw into an extension chord that has a sliding dimmer switch already built in (found at most hardware stores.)
For the truly adventrous, try an a/c powered vibrating device used in making concrete pre-cast products. That device is inserted into the poured forms and vibrates the concrete to prevent air pockets. This is an intense vibration that will rattle your teeth, but it DOES feel amazing, especially when massaged over your cock (pressed against your cock.) Enjoy at your own risk! BTW, the one I used was VERY quiet, given the intensity and power of the motor!