Hail all noble knights!
I am pondering this very question and the subject is creating a tournament helm. The decorated helm was but decoration and these helms were used in the opening and closing of a tournament field to identify the knights.
It was most common to use the animal from ones coat-of-arms with the metal color from the heraldry. The "cape" was a miniature of the edging and other colors from the coat-of-arms.
So lets say one has a great helm or sugarloaf, well one needs to find an animal that can be mounted -easy if your helm has a flat top, but not so for conical tops or movable visors.
Originally these decoration were made of wood wrapped in leather, or pottery, or even metal sculptures- as they were only worn for a short period, weight was not so much an issue. But can you imaging wearing this for a day at the faire? You would be sore.
So lets explore plastic, you can easily find plastic figures at toy stores, online and best of all educational stores that sell brands like Brio. Once you find a suitable figure, you can bend it using heat, or may have to cut parts, reposition and glue back together. If hollow- a block of wood (like oak or basswood) can be used as a mounting point then use minimal expanding foam (the one used around windows that will not buckle sills) to fill the figure and help lock in your mounting block.
You can then spray-paint your figure and mount it to your helm- here are a couple of things to think about:
A cloth “cape” usually goes under the figure and is made to “fly” about so it should be light material and generally would be made from two contrasting colors to match your coat of arms, with edging to match again that element in your coat of arms- like scalloping or embattlements. This also needs to be secured around the helm – if you have a steel helm- sew magnets into the boarder of the “cape” as this will allow repositioning and removal.
Next if you have a help with a visor….you will have to create a standard that raises the figure and allows the visor to open- not a big issue in an opening ceremony but as I think you may want to drink, eat and breath at a fair it is an issue here. Look at roman helmets for ideas here. It will need to be metal or alternately a light wood (balsa) or plastic cut and colored to work with your figure. This can be permanently affixed using bolts OR if light, go buy some super magnets and glue them to the base of your standard, keep adding until you can’t shake it off holding it upside down. Now its repositionable and removable.
Just where do I scrounge these super magnets you ask. Well any junk hard drive has two of these or you can buy them at hardware stores or craft stores.