I will give some dimensions and materials (available in UK) I use but obviously if or when you make yours just make it using what works for your own preference or available items. The general idea can be modified to make various shapes and sizes with the same basic approach, using plastic trays, I have made square, rectangle, a corner shape but I will show how I make round.

Sorry this may be difficult to follow, might be easier methods but hopefully should give you idea's at least.  I realise that everyone making model Ships are used to using solvents, glues, knifes and electric tools, well unfortunately making the "portholes" is just as dangerous, maybe more so with higher surface area coverage so ensure very good ventilation, an outside work space/garage, a drill, drill bits, jig saw, work horse etc would be good.

I personally cut the drainage connector in half slowly with a hacksaw and end up with two round plastic  tubes that only stick out 100mm into the room once hung as a picture.

 I use 9mm plywood, ensure you mark all lines for circles to cut, including the eigth holes before doing any cutting as easier to do on the whole board using the same centre.

Place the plastic round tube on the wood and draw all the way around the plastic tube on both the internal and external sides. The dimension for external side of My drainage tube is 324mm and I add 50mm for the width of porthole so wood width is around 425mm.  Once cut it should just slide over the complete length of the plastic tube.

Once all marked to a visually appealing round surround shape with holes, cut with jigsaw and hole cutter.  Less width and smaller holes, 15mm also looks nice.

nce you have cut out the surround you should have a round bit of plywood left over, the centre.  I use this as the picture.  It should just slide into the tube, if not cut until it does. 

Do not glue the struts into place as they need to be removed to place the finished picture board in once porthole all spray painted.

On the interior of the tube to give the background board something to butt up against and to hide the edges of the board I attach cork, using impact adhesive, and leaving a gap between the 3 wooden struts the size of the mounting board. Rough the inside of the tube with emery cloth.  I put the impact adhesive on both surfaces (cork and tube) to be joined, using as a first layer 1.5mm 70mm wide cork sticking against front edge around the inside plastic.

Next rough the external surface of the tube.  Then slide the cut plywood surround into place, position dependent on your preference and join the two together with Pink Grip,  ensuring holes symmetricaly spaced to the hanging center.  Put sealant weld on both sides of wood to hold it in place.

You can remove pink grip if your not happy with finish and then reapply either when when wet or after it has cured by cutting/peeling off, then redo. Pink Grip first thing I used and seems to be good at sticking the two materials together. Pink grip a tad runny if too hot but if temp less than 10'C does not grip plastic tube. 

Once this dried I then spray with enamel silver paint to give desired effect.

Once all dry I put a small bead of black sealant around the 5mm of cork on the inside, allow to dry, for the 2mm round picture glass to fit up against.

Personally I sand it, rough it, put on oil based wood primer, sand again, oil based wood primer and then do an oil painting. Give it a go, very relaxing in another annoying hobby kind of thing as you will then have even less hours in the day to do everything.  Once the background is ready to go in I nail around 7 small nails into the area where the Sea going to go, cutting off ends. Then remove the 3 struts, push in board – try to get it so horizon flat and level but do not worry if slightly out as you are looking through a porthole.  Put in the struts, drill a couple of holes and fit backboard to strut so it does not move. If you are happy with it then you can put pink grip around back or masking tape to stop spiders getting in, then put on a label and varnish back 

Do not worry if you want to change anything, just cut black sealant carefully and remove glass, then once finished put back in again.

If it seems a lot of work – it is, but effective as a picture and definitely different 

I am always in admiration of any one of any ability who has had a go at making a model and I wish all of you many thanks and best wishes if you have a go or get a better idea, Good luck and good modelling.