Ok, I re-read my other posts and we were talking about repairing the old tank. It sounds like now, if you've cut out most of the old one that you want to build a new tank and put it in?
If so, I'd plan on making an entirely fiberglass tank with no wood to go bad. It's a little more work, but you'll have no worries for the next 30 years. Measure exactly how big you want the finished box and then make a male mold (box) a little smaller. Screw a flat board/plywood on the open end of the box which sticks out at least an inch in all directions. This is your flange for the lid. For my project I'd round the corners well. If you want the box to "hang" on the stringers then you'll need to lay up the flange with more mat and cloth for strength. If you want the box to fit totally in between, then measure and build accordingly.
Sand and finish your mold to a nice smooth finish and seal with a couple coats of sealer. The smoother this is, the smoother the inside of your tank will be, which means easy cleaning. After your finish is dry and hard, wax it. Then wax it again. This will keep the resin from sticking to the mold.
Get your resin, glass mat and glass cloth at the local fiberglass supply house. Tell them what your project is and they will get you the correct stuff that will all work together. The basic idea is that the cloth is easy to make smooth and the mat gives good strength. The resin binds it all together. Listen carefully when they explain about making sure the mat is thoroughly saturated with resin.
Lay up your box using the instructions that come with your resin. The edges of the box flange will normally be something less than square and flat while you are soaking the resin in and laying the cloth on. No worries. Let the cloth extend slightly further than the finished flange will be. As the resin sets it will get firm to the touch. Now's the time to take a SHARP razor knife and a straight edge and trim the edges like you want them. The resin will also be a little tacky so use a scrap of plywood you will throw away as the straightedge.
When set, unmold it and decide where your inlets and outlets need to be. This would be a great place to get real fancy and use bulkhead fittings and all that cool stuff. I wouldn't be so fancy. Depending on your particular setup, I might just use a plastic thruhull in the lid or the side of the box for the shower to drain in, and let my pump hose go straight thru the lid and to the thruhull overboard. I think I'd have my electrical wires do the same thing, properly protected from chafing but nothing fancy and straight thru the lid. My thought is that if/when I needed to work on the system I could lift the lid and all the pieces would come out and set in the bilge. Or if the hose/wires were long enough, up on the cabin sole where I could actually work on it. Charles idea of mounting the switch and pump on something is a good one. Depending on which pump/switch combo you decide to use you'll need to field engineer that part.
For the top, I'd simply make a flat lid. This can be laid up on any no stick surface with the same resin/cloth/mat that the box is made of. An old piece of formica countertop works fine. Wax, then lay out what you need size wise. Something that might work for you, is marking the size lines directly on the formica or waxed finished plywood so you know that everything is within the boundaries. This can again be cut to size as above before the resin is totally hard.
I'd make "pins" to permanently attach to the lid for keeping it in place. I'd probably buy two or three 1/4" stainless bolts long enough to have about 3/4" of non threaded shoulder. I'd saw off the threads keeping the smooth shoulder area about 3/4 inch below the head. Then I'd grind them to a nice taper and round the end. I'd clamp the lid on the box and drill holes thru the lid and flange in three places that will be easy to see when the box is installed. Like maybe the middle of each end and the middle of the front. Take the lid off and drop the bolts thru the lid from the top. Glue them in place with more epoxy. Now the lid is fairly secure from being dislodged, easy to remove when needed, and easy to get back on in the right place each time.
Wash everything real well with soap and water, then paint with your favorite bilge color to make it look factory fresh.
I think that's the info you wanted. If not, just ask. I appologize for the delay in getting back to you. I seldom look down too far on the list.
Ken