Mounting a Double Hackbut on a Saddle

[Duriel_Jones]

I am planning on playing a mounted Musket Master in an upcoming game and I was wondering if there was a way to use the double hackbut and still remain on my mount.
In case you don't know the double hackbut requires a small cart to be set up(a full round action) to fire properly. If the stand is not set up you take - 4 to hit and are knocked prone. The double hackbut page goes on to say that a creature who is a size category one larger than the gun can shoot it without being knocked prone albeit with a penalty to hit for using an inappropriately sized weapon.

My question is that if the double hackbut is mounted on the saddle of a mount (which must be at least one size category larger than its rider) would the rider( or the mount) be knocked prone or take penalties to hit when firing the attached gun? I was thinking it would look something like this.

Also what about a medium mount with a small gun (and rider) mounted on it?

[Duriel_Jones]

One more thing,

[Silent Saturn]

If I were the GM, I'd say you could do it IF you bought a special "tripod saddle" for it, for (let's say) 300 gold. That way there's a trade-off, but once you've got it set up you can do it.