Controlling opponent with mounted kimura

[1PBCE1]

Specifically from the below position, what are the best ways of keeping your opponent there? I find they slip out from between my legs all the time. I see in the picture he's trapping a leg, I'll start experimenting with that, but other help would be appreciated.

 

[Russky]

Try two positions with your partner:

(a) Get Kimura grip and lift your partner into the air so his torso is off the mat and your back is straight. Then squeeze your knees together and sit on his head while still pulling up. You should feel very comfortable sitting like that.
(b) This is more like your picture. Get the grip and drive your forearm (elbow if you do not like this partner) into his back trying to flatten him face down on the mat. All your weight should be on your elbow.

Then your game is to switch between these two positions. You catch his arm and trying to finish (a), he grabs his arm/collar to counter, you push him down (b) break his grip and lift him up (a). Just do not forget to lift one knee and drive it into the shoulder to finish the armbar.