I needed to add a separate sump to our grey water system. I have a store bought sump from Rule in the forward end of the bilge for the head and shower, but needed one to handle everything else including it. At a $100 bucks or more for one of these plastic boxes, I figured I could make one for a lot less.
Here is how I did it. You can build one too. It's fairly simple. I must add I had some scrap Plexiglas already. I bought the 500 gph bilge pump on ebay. The rest was scrap pieces of marine plywood.
Box measure 11x11x6 inches. It is 1/2 inch ply with 3/4 x 1 inch around the inside for a lip to help seal it and give a place to mount the bolts. The plastic screen inside is actually from a craft store. It is used for some kind of needle work.
Build the box. Then give it several coats of thinned epoxy inside and out. Save money, Don't buy CPES epoxy, just cut your already mixed epoxy with some MEK (NOT MEKP), thin till it's just a little thicker than water, almost half and half mix. MEK is just like Acetone, but evaporates much slower. Let it cure between coats, but put the first 2 coats on while still green. Sand for the last coat and thin it only a little or maybe not at all. You'll know you have enough coats when it is very shiny when finished.
Side note; this little trick will help you fix rotted post and the like without buying special epoxy. I actually apply it to porch post and columns before installing them. Then they never rot.
I used 1/4 inch bolts for the lid. Insert them as you make the box.
Fit Plexiglas lid.
Install pump and float switch.
Install all the fittings in the box and screen plus the fuse holder outside.
Then install it. I eliminated 12 thru hulls with these two sumps. Now I only have 3 thru hulls.
I mounted it to the bilge with some stainless brackets I had. Screw them to the side of the box, make some pads that can be epoxied to the bilge , with bolts so that the whole thing is removable.
It is under the engine, but hopefully it will be somewhat accessible.