For an ideal end result, a cover that is customised to your barbecue model is of course preferable to a cover that does not fit perfectly.
However, there are also plenty of universal covers that, bought in the right size, work just as well as custom-made covers.
However, if you decide to make a barbecue cover yourself, it should of course be made to fit your barbecue as closely as possible.
If this is not the case, the grill cover can still get dirty and all your work will be ruined by the weather.