Mine's pretty much what I think you are looking to do. It's just a couple of pieces of ~3/4" stock, cut to abut the mast. If all you are planning is installation of spreader lights and a couple of pennant halyards like those shown here, I don't think any additional support's necessary. Mine has a wind speed anemometer as well.
Mine are stand alone. Aluminum,shaped right and welded to the mast. a halyard from the underside, out near the tip and a deck light on the underside, with the wiring inside.
When I got this boat it had an aluminum mast that was too short and had shrouds that cut through the passageway, so one would have to duck, or get a shroud across the neck. I replaced it all, saving the spreaders for the new mast. I had a taller mast made up out of pipe, lengthened the lifting boom so it would reach out over the water to the side, and mounted it without shrouds. I moved the base down a deck and attached the mast to the flybridge deck for support. That allowed me to use it to lift stuff, including a Laser that I carried for several years, and clearing the passageway eliminated the threat of injury while going up or down the steps. Aluminum pipe is strong enough and flexible enough for the job, without shrouds. When lifting anything heavy, I tie off from the spreader bases to a forward grab, just as belt and suspenders.
Find yourself a shop that will weld Aluminum and get it done right. You will never look back.
Here's mine - the mast came from a Katrina sailboat wreck - the guy who put it together welded clips to the mast that allow the spreaders to be through bolted to the clips - spreaders are fabbed from 4" channel, rolled to the ends to give a narrowing profile. The shrouds don't do much, if anything. It took 100 mph sustained during Isaac, so far so good.
When looking for lights, I wanted to illuminate the aft deck, but not ionize it. I found some Taco LEDs at WM.(TACO LED Deck Lights at West Marine) These are not halogen bright, but do the job. They periodically go on sale for about $35.