Fabrio, The "mini-boattails" behind my rear wheels are 30cm long and 12cm high. It looks like you might have room to install them behind your rear wheels. Even if you don't have the length to install a full-sized mini-boattail to block the entire width of your tire, you could make a shorter, reduced-width boattail to block part of the tire's width. That would be better than having nothing behind the tire to fill in the wake.
You might also consider installing double side skirts between the front and back wheels to help keep the air flow linear between the front and rear wheels.
I recently painted black my front wheel air deflecter, smooth hub caps, side skirts, and mini-boattail, so with the white front and rear wheel well skirts installed on my car and everything below that black so you can't see the wheels, from the distance my car looks like it is wheel-less and hovering.