I dont have a recipe as such but I think it you made up some sugar free jelly with chopped strawberries and raspberries (any fruit you fancied) inside, then layered with a light custard (im sure I seen a low syn custard somewhere on here before aswell if you searched) then topped with light cream , the syns wouldnt be that much per portion if you made a big bowl an then shared
I dont think there would be anyway of getting round the cake spongey part of a trifle but this would be similar :/