Just to add to Hanaldo's points...I regularly make one or two covers and the moulds were created using 1mm sheet wax over the original part

I tend to use two layers of 240g twill for the parts themselves, which gives sufficient structural integrity without feeling too flimsy, whilst not causing any fitment issues around some of the corners and edges. If the part to be covered has quite sharp angles and returns, you just need to be confident that you can achieve good consolidation of the chosen fabric into the mould corners so as not to hamper the fit of the cover.

If you prefer to use heavier backing fabrics, then you might need to take extra care to ensure that everything's tightly pressed into the corners to avoid unwanted bridging