Howdy rydan
board for some trial&error logic tests in a product we're working on. Problem is, it's an industrial product using 24V signal level,
Not clear if Arduino is used by you to test an industrial product of if Arduino will become part of an industrial product. If the latter, then its imperative that your solution be rugged and reliable and maintainable, in whihc case pubdc has the best answer. Really the only answer appropriate for an industrial product.
If you re just using Arduino yourself for in-house testing then many of the fine ideas expressed above will work. Took me a minute to find this but Grumpy_Mike provided this to another post, its relevant to you too if you wanna "roll yer own" rather than use standard industrial I/O like opto-22.