if you absolutely have to have inners, I used to make em out of a piece of 1/8" ABS plastic sourced from Mcmaster.
Heat the stuff up with a kerosene heatr till it's good and wobbly soft.
It will be super flexible and you can mold it with your hands(Use gloves)!! clamp it in place with some spring clamps along the wheelwell edge and mold it to the shape you want with your fist, and hit it with the air hose to cool it.
It will retain the shape.
Any minor adjustments can be made by reheating with a heat gun, and remolding with your hand
Just don't cook it, cause it looses it's elasticity.
I used poprivits along the inner and outer edges to hold it.
Only thing is I'm not so sure that stuff will take the extreme cold, but we don't get that where im at.
I also know some guys who made crappy ones from a cut up cheap green garbage can from Home depot. kinda ugly, but very flexible and they worked! Bottom side of the can goes inside and trim to fit around whatever. cut em with a knife .
Hope that helps!:yelclap: