The easier method is to use one of those 4xAA batteries holder, it has 2 wires which you can stick to the terminals inside Stampede battery tray. I use 4 TrustFire batteries to power my SgNerf's spring and it works well. You can check out SgNerf website for his mod guide, very useful.
My second Stampede has Servulus's spring which the same 4 TrustFire batteries are not strong enough the prime it. So I ordered 6 AA-D battery converters from DealExtreme, put in 5 TrustFire batteries and the sixth one has just a dummy AA battery in it. Beware of the AA-D battery converters you buy, those I got from DealExtreme actually hold 2 AA batteries in each converter, so I have to add a dummy AA battery (without wire contact) in each converter so they sit correctly inside the converter. There maybe other AA-D converters that only can hold 1 AA batteries which is less hassle.