Attach a strip of hardboard with double stick tape and follow the bottom edge of the pattern.
Attach a second strip of hardboard the same profile only 1/2” apart to create a "trough" for the 1/2" guide.
Use a 1/2” brass bar or a 1/2” guide as a spacer. Also use 1/2” wide strips of hardboard as stops at both ends of the pattern.
Use a 1/2” guide and 3/8” bit in a portable router to cut the slot in the pattern.