Had same problem with my IBC , I used a 9inch inspection hatch from Barton marine ,they use em in boats and yachts for covering holes . They come with a separate flange and a large threaded white cover , cut out hole where the original lid is ,bolted flange in with stainless nuts and bolts ,screw on cover and job done.
Price was £15.00 but its good quality gear. If you type in inspection hatch it should come up,makes neat job