Right now you can get locking Spyder feednecks for only $16 from ANS gear, and non-locking ones for just $3. Paintball - Cheap Paintball Guns, Gear and Paintball Equipment
The downside is that they're Warrior brand and mismanufactured slightly- the locking axel is 8-32 but the included screw is only 6-32 and the rest of the neck is manufactured accordingly. You have to get an 8-32 machine screw of the proper length and drill out the aluminum portion of the neck to get everything to work right. That's in addition to the usual work to get a Spyder feed to fit correctly, mind you.
Oh, and I didn't have to modify the Phantom body at all, only the feedneck.