Broad leaved plastic or real plants.other than that a strip of slate about 2" wide 1/4 or more thick and at least 10" long is best.Lean it in the corner or they will knock it over and get hurt.
My angelfish ignore the slate and lay their eggs on the decoration. A flat surface at the center of the tank should work. I don't think they care about what they lay eggs on as long as the site is clean enough so they can clean it.
Any clean, flat surface set at an angle should invite angels to lay their eggs their. This is only relative. I have had angels ignore plants and lay on rocks and vice-versa. All you can do is experiment and find what is best for you.
aside from large water chnges and an elevated temp and prep foods find a nice plant they like or a smooth flat surface ive seen eggs on a variety of surface its more of a comfort thing than a specific surface type