As long as the water injection point is a foot or more above the waterline you don't need a siphon break. You can build a perfectly adequate riser from black iron pipe and fittings from your local hardware store. I would suggest a Centek fiberglass water lift muffler instead of a Vetus for two reasons. First the Centek is made of fiberglass and high temperature resin. It can take a bit of over heating a lot better than a plastic Vetus. Second the Centek mufflers come in a wide variety of shapes and sizes and are cheaper than Vetus. As far as sizing the muffler is concerned, it should have enough volume to hold all the water in the hose above the muffler. If you have five feet of hose above the muffler a one gallon capacity muffler is all you need. Even that will give you a safety margin since the hose above the muffler will seldom be completely full of water and you also have the volume of the hose up to the injection elbow.

As long as the muffler is below the injection point there is no particular reason to make it as "low as possible". Making the muffler lower simply requires a larger muffler. That said, having it lower does provide more safety margin.

Here is the installation on Tortuga. The muffler is behind the bulkhead on the left. The water injection elbow is about 13" above the waterline.

The picture shows the Centek Vernalift muffler I put on Tortuga. It has 2" hose. This is the side in, top out model. They also make top in - top out versions. The injection point for this installation is only about 14" above the inlet of the muffler. I do not have a siphon break. I do close the raw water seacock when the engine isn't running.