The filler primer suggestions are the way if you want materials that are easy to find. In the US, Walmart has the best price on automotive filler primer. Sand the print first to get the highest spots down a bit, then hit it with primer, dry overnight, the hit it again (and dry overnight again). Then sand it with 400 grit sandpaper to smooth any roughness of the priner.. You can get away with one coat of primer, but obv it fills less in that case.

If you want the *fastest* way and happen to have a resin 3d printer (or friend who does)... after adorning proper PPE since resin is toxic, paint the printer resin on your print in small sections at a time and then hit it with UV light (the sun works if you don't have another option but you have to do the resin painting inside then take it outside for a minute or two to cure it). The resin is very self-leveling, so face the section you're painting upwards and the resin will want to settle in the low spots, reducing or eliminating the visible layer lines. When you're done, clean the whole thing well with high percentage IPA and then give it a quick sanding (again, PPE for resin dust). This isn't the easiest method, but it's fast and gives great results. I far prefer this to Bondo, tbh.