The J-Stim has been tried and tested, It inserts in to the vagina and the natural curve makes it push upward on to the G-Spot.
Be aware that it is quite likely to cause a female to squirt when used correctly.
Printed at 0.2 layers, 6 wall thickness and 25% infill to ensure the shaft has the strength. Best printed in ABS.
The unit can be scaled to whatever size you like however I found creating the head to be about 35mm to 40mm seems to work quite well.