This thread deserved more interest. I'm really not sure what you mean in the second paragraph though. Hoping you're not referring to a bug deflector. I am assuming you're talking about the first gen RL but I would like to finally see a discussion about the new one on this forum.
Step 1: Remove mudflaps.
Step 2: Add an aerocap. This has not been done before, to my knowledge. Probably because Honda marketed it as the most aerodynamic sub-full-size pickup out there. I am working on this step currently for my truck. I'm making it out of polycarbonate and conduit. First gen Ridgelines take to the "Optimal 12°" angle from the Holy Thesis pretty well. The end has 1.5"-3.5" separation from the tailgate, minimal, which imo looks better than a large rear gap like some toppers have. Also, with any RL you don't have to worry about rear lift from your ecotopper because it switches to FWD over 15mph
Step 3: Weep about The Aesthetic and remove the pretty roof rack many of them have. I may skip this step, or just do the cross bars, because I love the look of a good OEM roof rack on an AWD vehicle.
Step 4: Tape the few seams. Possibly add a cover over rear wheel wells. I would skip the moonwheels because the OEM wheels are already mostly covered and a vehicle that tows should probably worry about brake heat.
Step 5: Partial grill block. Again be careful with this if you intend to tow. I may add one this fall.
Step 6: Belly pan? Same heat concerns, and these are the trickiest part to pull off correctly with actual Cd savings because of all the vortex junk ya gotta look out for. I'll probably leave this alone, myself. Alternatively, you could have a front bumper lip, but those just seem out of place on a 4wd vehicle imo and they're hard to get right.
Step 7: After you mess with the underbody vortex junk you might as well boattail the tailgate. A plus for the RL compared to other trucks is that it swings to the side, so you could tack on as much as you want to the tailgate then just never open it downwards lol.
It should get +3-6 better mpg out of all of this. With no mods and some mild hypermiling I get 20 mpg for both city and highway. I'd love to see what the gen2 would get, but I can't afford that. I bought an RL because they became inexpensive right after all the fanboys traded for the new one! I'm gonna start modding as soon as the semester is over (I'm a MechE major).