Design Suggestions Please

I am designing a alignment block that will slide on a rail carriage. The block has a slot to locate a flat on a part. The critical feat of the block is the height of the centerline of the part. I would like to make the height of this block adjustable for the user, so that if the centerline for the part changes, they can easily adjust the height a few mm.

I don’t have a lot of experience in design or in general. I am 21 working my first engineering internship. Can anyone offer some help on an easy way to make the height adjustable?

Thanks!