Your kicks don't have to be your rear leg in your stance. Check out the switch kick. Not sure exactly what it's called but you basically just switch stances fast and deliver the opposite kick right away. I came about it when after I watched a video on Bas Rutten and how he prefers body shots, mostly his left, the opponent's right, side, which is a liver shot. What do ya know, the next day boxing with my dad, I take a wicked lead hook to that area and am completley rocked. I thought to myself, "I have to get an efficient way to strike there besides just a lead hook." I did a little research and found the switch kick. I personally find it easiest when I check one of my opponent's leg kicks, then bring the checking leg back and throw the round kick to the liver. Devastating and simple in the two or three times I've used it.

For you, you could switch then send the leg kick. For one, the switch throws them off. Also, against a southpaw, they won't be expecting a leg kick to the outside leg. I also just saw a video of Dr Sick's that involved a switch jab or something, where you throw the jab and switch your feet at the same time. In his case he was describing against a wrestler who's going to wait for you to punch, then they will shoot, then you throw the knee. I (not sure if I'm correct) could see how you could jab (or straight right in your case), switch your stance at the same time, and while their guard is focused on not getting snapped by a jab/cross, throw that opposite rear round kick. It's going to take some practice and drilling. It took me a whole month of nearly daily drilling before I even felt comfortable with the switch kick, or a round kick of the opposite side. Good luck bro.