i would not recommend the expandi-foam method above. dont know if you ever used that stuff, but certain brands are damn near impossible to get off.
for "easily removable" back- get some dense foam, cut it slightly larger than wheel opening, stuff it in there
honestly for the fronts, i have no idea, dont know anyone that runs mud plugs on front wheels
back- cut a piece of plastic ( i would use something not completely rigid so you dont cracking it) out the size if the wheel, drill holes to match beadlock bolts, remove beadlock bolts, bolt down ( could run bead of silicone around to make it practically water tight, but i dont see the point as you still have hole around hub.
Front- could do the same thing if those outer bolts are removeable.
hell, you could combine the two, stuff foam in, then bolt on a plastic cover, foam inside would keep some mudd from coming thru around the hub.