An Arduino is simply an ATMega chip wired up with support devices. If you can get it on a circuit board that fits, you can do it yourself and learn something in the process. I am now forever putting ATMega328P chips on my project boards with a 3x2 female header and just programming it in place with an Atmel Dragon. I use the Arduino software and then burn it using the Dragon and the software included with Atmel Studio. Here is some more information for a minimal configuration, it takes at ATMega 328P chip, plus a switch and a 10k resistor of you want a reset, plus a crystal and two capacitors if you want to run at full speed:
If you want a real lightweight configuration, consider running it at 3.3V and use a supercapacitor to power it. Not sure how long it would run, but maybe it's a possibility if this is a rocket or something like that.