I have looked at a number of mounting post concepts, which I have used in my bdsm fiction writing.

On the safety issue, there can be a serious risk of rupture and impalement, if the basic concepts are not properly understood, or incompetently applied.

Firstly, it is essential to have some kind of flange or crossbar through the pole to make sure that the dildo part cannot penetrate any further than is safe. The flange or crossbar must be very securely fixed so it cannot slip. Something like a bicycle saddle could be used. Securing the hands behind the back is sufficient to ensure that the woman cannot climb off.

Secondly, the length that is inserted must not exert too much pressure on the vagina or the cervix. The pressure is inversely proportional to the cross sectional area of the shaft. The thinner the shaft, the greater the pressure so conversely, a thicker shaft is safer.

Thirdly, a good way to ensure that there is no risk of harm due to excessive penetration is to secure the woman in an upright spreadeagled position, her arms holding her up so she can't sink down too far. Always remove the post before releasing her arms to make sure she doesn't slip or fall onto it.

Lastly, however it is done, don't ever allow the woman freedom to attempt to climb off unassisted after she is mounted, because if she slips she could be injured. Don't leave her unattended for long periods either.

Once she is spreadeagled, I like the concept of a thick wooden mounting post with a smooth rounded top, spring mounted in a cylindrical sleeve, the spring allowing 6-8 inches of vertical movement. You adjust the post so that with the spring compressed, it presses up into the soft hollow of her vulva. Her task it to resist penetration by clenching her vulva muscles as hard as she can. This is an excellent pelvic floor exercise. With plenty of natural lubrication, encouraged by the partner's teasing fingers, this can become a difficult challenge. A few strokes with a cane or other suitable punishment implement will almost certainly cause her to clench and relax for a moment and before she can stop it she is well and truly mounted. After that there is plenty of scope for play, and lots of lovely orgasms.

One of the variations I looked at was varying the roundness of the tip, making the cross section more elliptical to increase the difficulty, or more spherical to make it easier. Using wood all the way up means that you can use a larger diameter, if she can take it. 1.5" to 2" is probably ideal. I can send people a spreadsheet that shows the calculations for the tip profiles. Contact jolan2 at yahoo.com for the spreadsheet.

There are several mechanical alternatives to a spring mounted shaft, using pulleys and weights or even hydraulic pressure. The important thing is not to use something too forceful like a geared motor. There needs to be a certain elasticity, rather than rigidity in the way the pressure feels.

Vaginal mounting opens up other possibilities for bdsm play including the insertion of suppositories, enemas and even anal sex while she is mounted, not that I would encourage any of the above!