This adapter is made to slide into the place occupied by the drums, while still being removable.
Mods to blaster required: remove the orange dart stop tab in front of the flywheel cage. Add a stronger return spring to the pusher arm (it will have to retract while under force from a Nerf magazine).
Print adapter module, remove extra pieces after printing. Print the actual magazine holder halves and locking tab, and glue/screw together as desired. Then glue that assembly into the adapter module, using a magazine as a guide.
Magazine halves are remixed from a previous Thingiverse post and use M3 bolts. The magazine release tab will need a small spring.
UPDATE: I flipped the Adapter module file to allow for left-handed use.This will, however, require the user to remove the drum advancing tab, as that will otherwise block the use of a magazine in that orientation.