In the tie-down method, you can fix large puppets or heavy puppets, but the load on the foot is quite large. To that extent, you need a solid work that is strong.
Make the framework of foot with metal parts and cut the threaded screw hole on the soles of the toe. when walking, the foot rise from the heel then the toe will leave the ground. so it is common to drill a hole in the toe side.
To cut a threaded screw hole, use a tool a drill and a tap to cut a thread.
If processing metal is difficult, you can also use a method to embed the nut in the foot. The nut is grued with epoxy adhesive or embed it in the foot with epoxy type putty.