custom decals

A way around the problem mfsob pointed out might be to put the design on a background color that would work for the entire interior surface of the gunport lid, and have the decal cover the whole surface.

You might also take a look at the line of dry transfers sold by Miniatures Andrea, of Spain.  They produce a fairly large range of heraldic subject for their medieval and Renaissance knight figures.  Andrea is a fine company; the samples of their transfers that I've seen are beautiful.  Several of them include fleurs de lis - though finding an appropriate size would be a matter of luck, and the price for the necessary number of sheets would add up. 

Squadron Mail Order ( www.squadron.com ) carries the Andrea line.  Go to "Search," then "Decals and solvents" / "All scales" / "Andrea Miniatures from Spain."  The French, English, and generic heraldic sheets are worth a look. 

What kp80 is referring to is the Testor's Decal Making Kit (think that's the correct name). I picked one up at my local hobby shop for about $9, and it comes with one sheet each of clear and white decal film.

BUT - white decals are a problem. There is no currently-produced inkjet printer that will print white. You might try printing a very, very light gray on the white decal film, but you would have to cut out each tiny little fleur individually. Printing them outlined in black would present the same trimming issues. Or you could try very light gray on the clear film. However, I have had problems with light-colored decals going translucent (invisible) when put down on a dark surface, in my case an olive drab airplane.