Retrofit overflow box with or without back?

[Weapons462]

I am replacing ABS bulkheads that go through the back of my tank. I bought this setup second hand and I think the box inside the tank is custom. It is very narrow and I will have to pry it appart to get the old bulkheads out. Not sure if I can do it without breaking anything so I plan on having another box on hand.
My question. What are the benefits and drawbacks for a 4 side box compared with a 5 side box. Seems like the four side with the bulkhead sealing directly to the tank would be safer.

[ca1ore]

I personally prefer the 5 sided boxes only because acrylic does not adhere with silicone to glass all that well and I have knocked a back-less box off the back wall before. Reattaching then becomes a problem, not to mention the possibility of overflowing the sump. With the back (fifth side), the surface area for silicone is greater, or the bulkhead just holds it firmly in place. If you don't silicone the skim box to the back wall, an additional gasket is required, but the water pressure is not particularly high and they seal well.

[Lsufan]

I agree with everything calore said but they do make shorter bulkheads in some sizes. So u may be able to cut out your existing bulkheads & replace them with the short ones. That may be able to save u from having to replace the box.

The only way I would use a 4 sided acrylic box on a glass tank is if I made the overflow out of glass & just wrapped it with acrylic for the look.

[Weapons462]

Thanks folks. I didn't think about that. Thanks for the information!