Just an idea, that I have used earlier.

Use a stick, a pen or another round cylinder object that you can hold in your cunt for a certain amount of time. the more smooth and thinner the object is, the more difficult it will be to keep it up. At the same time this will train your muscles in the cunt and make you even more tight for him.

Again this can be made more difficult by spreading the legs more. 1½ feet is a good starting point. Adding weight will enhance the task even further.

Best of luck

SirRopes