A through the back wall bulkhead is cheaper and more dependable than any and all overflows, bar none. Even with the purchase of bit, bulkhead and all piping and fittings you will have less money invested than just the cost of a fair, not good, siphon overflow. And an overflow with pump assist costs even more. Glass holes really are easy to drill! Really. Hell, you could put in two back wall overflows and still have less money invested and I have never heard of two separate bulkhead overflows getting clogged up at the same time. Have some guts people, drill holes, like professionals, quit being so amateurish! Take a chance. Win a prize. Like a good dependable overflow!