OK. First, you buy a fibreglass kit like the one above and some silicone release spray. You'll also need some wet and dry paper to rub it down.
1. Make a wooden frame the shape of the gap in the frame. It needs to be a few mm smaller than the gap all round to allow for the thickness of the grp. You can use epoxy or PVA glue to glue it together. It needs to be a bit thicker than half the thickness of the finished box.
2. When you're happy with the shape, glue a piece of 3mm plywood to it that just overlaps the frame all round.
3. When the glue is dry/set, use a plane to round off all the corners, then sand it as smooth as possible.
4. Use it as a pattern to make the other side, so you have two identical mirrored ones.
5. You need to add some wood filler to any gaps in the wood, especially where you've sanded the edge of the plywood. Sand everything very smooth with 320 grade or finer.
6. Wax polish it so that it's slippery to touch. Finally, spray with the release agent. Now it's ready.
7. I used woven mat rather than the rough mat in the kit because it gives a flatter finish. You cut a piece of cloth big enough to cover the wood, lay it over the mould, mix up the resin with the catalyst and paint it on. Don't use more than enough to make it wet. You can see any air bubbles, which you can poke with the brush to get rid of them.
8. Lay over another layer if you used woven cloth. One layer is probably enough for the rough stuff. Add two more layers of the finishing cloth. Leave it to cure. It's best to precut all the cloth because the resin starts to go thick after about half an hour. Once it starts to go thick, don't use it. Mix some more.
9. When it's cured, you can remove it from the mould. It's probably well stuck. You can use a dremmel to cut the corners so that you can peel back each edge, which makes it easy to get off. Don't worry about cutting it because it's easy to repair that afterwards.
10. Once it's off, you can repair the corners. New layers won't stick unless you remove the waxy surface, so wherever you want to bandage the cuts, sand it all around. A Dremmel is really good for that. Make up some more resin, cut some bandage strips and paint them on inside and out - more on the inside to add strength to the corners. When they're cured, you can sand them down on the outside to make them nice and smooth again.
11. Use wet and dry to sand it all over smooth.
12. Hold a pencil on a block so that it's the height of half the thickness of the finished box. Don't forget to allow for the thickness of the grp. Run round your box to make a cutting line, then trim the edge to the line. For that you need some good snips or an angle grinder, or any other tool you can think of.
13. If you want it to look really nice, paint it with high-build primer, rub everything down very smooth, then add your final paint.
14 To fix the two halves together, I used 5mm thick softwood. Cut some strips slightly less than the length of the sides and wide enough to get a good overlap. Glue and screw them to one side.
15. Mount that side to your frame. You need to drill holes where the bottle fixings are. Use washers on the inside to stop the screw heads from digging in to the wood. You can design your own brackets to hold the top. I used a single U bracket at the top with one side bolted tight to the box and the orher side loose with a nyloc nut. I then cut a slot in the other half of the box so that it could be slid on. Once slid on, you can screw the second half to the wood.
That's it. Don't worry about any small gaps in the corners. Any water that gets in will run down the bottom edge and back out the bottom corner. Make sure that the way through isn't blocked. You can seal ot all up if you want, but IMHO it's not worth the effort.
