How do I Make a "Custom mob spawner block"

Set its tick speed to the number of seconds you want for the delay times 20 (so 30 seconds would be 600 tick speed), then on an Update Tick procedure have it spawn mobs. You can't get it as advanced as Vanilla spawners (they wait a random amount, and spawn the mobs randomly in a certain area, with all sorts of conditions), but you can have it very basic (such as every 30 seconds, spawn 1-4 zombies on top of it).