Print flat and assemble. I'm including a pdf of a sample card you can print on a regular ink printer... then insert the 3D print and send it to your Valentine :-)
UPDATE: I've added 2 versions of the base, one with 0.25mm clearance and the other with 0.5mm clearance. If your printer prints exact, use the 0.25 file(s). If you have trouble with prints being too tight, use the 0.5mm file(s). Note that the only thing that changes between the two is the circular base, all other parts are the same.