Well...
Yeah, I could put an X behind the front seats but...
I milled over that in my mind for a long time. An X brace there would really cramp the style of anyone in the back seat. It just doesn't leave any legroom. Originally, I didn't have the X on the back, but later added it as a compromise for not having it farther forward. The way I thought about it (and I've never been accused of being the sharpest tool in the shed) was the roof is U shaped in the back to follow the lines of the frame surrounding the engine. This seemed like a weak point to me since the roof bars aren't bent downward in the back and attached to the frame like most buggies you see. I wanted it to be as strong as possible back there but I also knew I needed some sort of X-ing. You may also notice that there are some gusset-type grab bars near the roof for the rear passengers. They aren't there by accident as they serve a somewhat X purpose as well.
Now I know it's probably not ideal, and I doubt anyone will want to copy it, but I tried to come to a happy medium by taking into consideration the use of the rear seat. It's not easy to put a functional rear seat into a 2 seater frame. There has got to be some compromise somewhere and I really wanted to be able to fit two adults in the rear. I'm trying to get my cousin into buggies and that way I can take him riding and have the ladies in the back. If it were for kids only I could probably do it, but I want my cake and eat it too.
I appreciate the comments and suggestions. I hope I explained my thinking in a way that somewhat makes sense and not like it was just thrown together. I could go ahead and add the X where it really needs to be and really limit the use of the back seat, but if it's not going to kill me I'd like to leave it like it is.
Thanks fellers, yall know way more than I do about this stuff. Keep it coming...