I think probably the best option is a sex machine. Depending on which one you get, you can change the angle on it and it won't be too bulky, so it won't get in the way of your tripod. I know I'm eventually going to take the plunge and get one (or maybe even two).
But for the time being, I either use my hands and just hold the base of the dildo right out of the frame if I'm doing an above, looking down POV or I put it on the bed and just make sure the base isn't in the shot for a laying down POV. I've also used the bottom for a mannequin for a few videos involving facials, where I needed a free hand. I remove the top of the mannequin so I can position the camera over it and facing down, then just suction it to the mannequin's groin area.