Ok Les got my flies out and had a look. Here are some pics to help you choose.
I also lined up next to it for scale a ruler in inches and my wood pen which Mack Cameron made for me. Just for sizing this pen is about 1/2 inch diameter. But you can also take the dimensions of the pen from the flag kits that are available as this is the Canadian Flag pen.
Mack will be able to tell you exactly all the details of which kit he used if need be for sizes.
So i got a double hook salmon trout fly. I had originally felt that this could be cast such that the double hooks are bent in a way that it staddles the barrel of the pen.
You can see this type of fly is colorful and typical of the fly that people associate with the sport. I have shown it on a ruler scale in inches so you get an idea of size.
Then here is a pic overview of my box of flies. Loads of them. All sorts of sizes. I aint too good with hook numbers
even after all these years. Its a stupid numbering system imho no dimensions all relative.
Anything you want i will send it. We just have to get the size right first.
Here is a the pen and flies and one that is a single fly.
Then i took a small one and used tape and stuck it on the pen barrel of the Canadian flag pen so you could see.
Just doing this made me realize if you use a fly like this you going to have to sort of cut it in half so it lies flat cause the feathers span out around the head 360 degrees. A simple cut with the scissors will flatten it and i reckon a little super glue will hold it on your base barrel while you cast. This would be real neat if you can get the feathers to stay pointing up while the casting material flows around it. Give it a sort of 3D look. Like looking through the looking glass.
So I make the hook on this one to be 3/8 in length and 3/16 in hook width.
I have samller but then they get so minimal that it aint great to look at unless you getting some magnification effect with the cast clear material
So you decide i have no clue on the casting and what is doable. You might need to think of a different pen kit. I cannot advise you in that area i aint a pen turner .......yet
Just thinking aloud and trying to get out the box, i would think you would first cast a thin layer of material like say white around the barrel of the pen. Then mount the fly and get it to stay put, heck maybe some mono filament would do this. If fish cannot see it in water, i mean the real thin stuff we use for tippet in flyfishing, then dont see why it could not be used to stop the hook floating off the pen barrel. Then cast again with clear.
If the salmon hook with a double hook was used and "flattened out the height aspect would only be the thickness of the feathers and the hook shaft.
I am prepared to post you anything you want to give it a go if you like. Heck its only feathers and bit of steel. I wont use up all these flies with the amount of time i get to fish.
Choose the small brown one to match your feathers.
So balls in your court Les.