escort cab illusion kit

[jim05_367]

can anyone help? ive got a escort cab with an illusion kit. the problem ive got is that the filler on the rear arces and skirts are cracking round the joins. ive been told that it should've been fibreglassed first then filled for strength but i would loose the straight edges and it would look bloody silly. does anyone know of anything to fill it with that is strong enough.
many thanks, james