You can make on very easily and save about $60.00 after you add the cost of material. check out this site.
DIY Overflow Box
Here is the one I made and it works grate.
I did not use a pump I just glued a piece of ridged air line and put a air line to it with a check valve and I just give it a suck when I see air building up in it about 1-2 times a month. as for bulkhead size that depends on how much flow you want. Here is a site that will give you the answer.
Reef Central Online Community