Really need those PLA parts to be truly stuck together? Or perhaps you need a giant 3D printing pen? This "glue stick" can be printed out of PLA and fed into a standard hot glue gun.
I recommend at least a 40 Watt glue gun to ensure the PLA will melt, the heatup times will be longer than hot glue. Also, the glue stick needs to be printed with 100% infill.