You will reduce flow with a series of holes rather than slots also it will be difficult to clean, not a good method IMO. They can be constructed without slots or holes with flow over the top. I would look into buying if you don't have the ability to make one.
You make a series of horizontal holes with a drill at the intended water line. Since I assume it's acrylic or polypropylene, you can force a higher speed drill to turn them into slits like some of the ghost overflows. If you take your time you should be able to make it look decent. Just start with smaller drill bits.
Or just keep it slotless. By skipping the slots you get a thinner surface skim which is preferable. (I like slots, I always use slots on my overflows, but from a design standpoint slotless is ideal)