Like the way you think. Any photos?
Any truck size mudflaps will work. But if you can find a large cheap "stiff" mudflap with no writing on it (or plain black mud-flaps), they are the best blank-slat you can just cut with sharp scissors to any design you want, without being restricted by what is printed on it already.
Then just attach with 1/4" self-tapping screws spaced out (the mudflap directions should indicate the number of equal-spaced screws and suggested size that are required) attaching it to the inside of the wheel-well. It's a good investment for about $20 at auto parts store. You may have to search a couple of different auto stores since sometimes they stock the 'floppy' mudflaps that don't do any good at all.
Cut it up to 1/2" beyond your wheel width, for aerodynamics, and as close to the ground as you can. It may not be too close to the ground, but it will be better than factory molded mudflaps: and factory molded mudflaps are better than nothing. Any solution works to some degree, and usually, the best is the cheapest too. However, if you don't get a flat, factory molded flaps are as good at the theoretical best coverage, except you pay 4 times as much. But they may be more aerodynamic: so if you don't want to mess with cutting the mudflaps, it is a convenient rationalization I use when i'm lazy