My CRX doesn't have a belly pan. However, the factory design made an attempt to partially help the under body air flow, by attempting to smooth and contour the 1st foot or two of air space on the front underside of the car. This might not be as good as a full belly pan, but in theory at least it should give a decent partial benefit.
Of course, to make it easy to work on the underside of the car, they used a number of smaller (aprox 1 square foot, but they were all irregular contoured pieces) black plastic sections individually bolted on (instead of making one big "belly pan" that had to be removed as a big piece).
Well, my mechanic noticed (when he was otherwise working on the underside of my car) that many of those plastic sections had relatively large gaps (sometimes a centimeter or more) between them and the adjacent section. Furthermore the edges of some of these plastic pieces had bent over the years, making them more likely to catch the wind. Since he knew about my quest for FE (and aerodynamics is a part of that), he pointed out how these two facts could hurt aerodynamics (even though he didn't know what would be a good fix).
So I got to thinking. What could I do to "fix" this? I thought about caulk, but rejected that idea because it would make it hard to work on the car (i.e. you would have to constantly cut the caulk and re-caulk everytime you wanted to remove a plastic section). So what to do...
Then, being a fan of the "Red Green" show, it hit me. This is actually one of the few times when "Duct Tape" is actually the correct answer!
So I got a role of black backed duct tape (so that the tape color matches the plastic pieces it is going over), and started putting duct tape on the seams between the plastic pieces under the car. With the duct tape, I could both cover over the gaps between the plastic sections, as well as "tape down" the edges that were curling up (and thereby catching the air). And whenever someone needs to remove one of those sections to work under the car, you just pull up the duct tape and then unbolt things normally (and then just put a little more tape down on the seams, as soon as you bolt the plastic piece back on).
I've only had the duct tape on for a short while (a week or two). And I don't have real time instrumentation, so it's hard to quantify the effect. However, subjectively I noticed the improvement in the car's aerodynamics immediately. In fact, my subjective guess is that this "duct tape mod" (under the car's front end) made even more of a difference in improving aerodynamics than my coroplast grill block previously had (which is not to say that I intend to remove my grill block, only that I think my duct tape under my car helped even more than the grill block did)!
In any event, I don't in any way feel bad about putting duct tape UNDER the car just to improve aerodynamics, especially since I have the tape color matching the color of the plastic I have the tape connected to (thereby making the tape almost "invisible" unless you carefully look under the car for some reason). The way I figure it, duct tape is cheap, easy to use, and easy to remove (especially from the plastic it is taped to) when I later need to remove it (to make repairs to the car). And so what if the tape eventually falls off on it's own? If/when that happens, I'll just get another roll, and tape it up again!
And at least from my perspective, it really "feels" like I've made a noticeable improvement to my car's aerodynamics using nothing more than "The Handyman's Secret Weapon"...