Suomalainen means "Finnish" (A person from Finland)
Ilta means "Night", but -lainen doesnt work with it.

Im not sure what word you want to create, please use it in a sentence so i can understand the context ^^.

"Iltainen" is the correct derivation, but it depends on the context whether you should use that or the genitive form "illan".

Examples of "iltainen":

Tämäniltainen elokuva on parempi kuin huomisiltainen. The movie of this evening is better than the one of tomorrow evening.

Iltaisen kadun valot heijastuvat tummapukuisen miehen aurinkolaseista. The lights of a street of evening are reflected from the sunglasses of a man dressed in black.

Naapurin vauvan jokailtainen itku häiritsee minua. The nightly cry of the neighbour's baby is disturbing me.