Unibond works, as does Smooth On.
I often stack the two slats and clamp them, with the pattern glued onto the top one with spray adhesive. After the splice is cut, you simply unclamp them, flip one over, and slide them together.
It takes just a few short minutes to create a perfectly mated splice... so no need to mess with jigs. Splicing doesn't get any easier than when done to 1/2" slats.