I decided to list our banners at https://heyuri.net/index.php?p=advertise for other websites to use when linking to Heyuri.
...but we seem to have a lack of banners

How u decide to design is up to you, but some cute 2D girl (I think Japanese often draw such CG themselves, but it should be fine to get them off old booru posts or 2D Cute board; or our own higher quality oekaki pics) and a pretty font is what a lot of Japanese otaku-muke sites seem to do
I couldn't find a good collection of them on the internet, but you can view a few examples @ https://m-pe.tv/u/page.php?uid=99tdtd8532&id=13 or https://moeeki.net/rori.htm

200x40 is the one I know that we're lacking, but feel free to submit any other commonly used dimensions, they'll all be listed on that page ヽ(´ー｀)ノ