Studying shapes of this article: https://www.punipunijapan.com/shapes-in-japanese/
And noticed only some shapes have adjectival forms.
What is the proper way of saying stuff like Octagonal?
I'm thinking either ShapeのNoun, Shape的なNoun, or possibly even ShapeみたいなNoun but this one feels wrong.
Yes, only a few words (丸い and 四角い) have i-adjective forms. For others, you can treat them as no-adjectives: 八角形の (octagonal), 星形の (star-shaped), 円柱形の (cylindrical) and so on. (Don't forget 形; 星の would mean "starry".)
Also note that 丸い and 四角い are suitable in everyday conversations but these are not suitable in mathematical contexts. See this answer. The "technical" version for 丸(い) is 円/円形の ("circle/circular") or 球/球形の ("sphere/spherical").
八角形的な and 八角形みたいな would mean "octagon-ish" (i.e., something not exactly octagonal).
