A quick look of my sources shows only one example of a 40th Miss. Inf. uniform in a photograph. An officer. He is wearing a light colored short brimed slouch hat.

Looking at other Mississippi units, that fought in the West, you'll find a plithera of head gear for enlisted. The only examples of the "bugle" used was either in brass or embroidered on the Hardee Hat. The kepi style is worn plain, or worn with a color identifier band (some with the front of the color band peaked) being a dark color like black or navy blue. The kepi comes in various colors, light grey, dark grey, dark blue, or butternut. One example seems to be a black kepi with a red band around the bottom (not peaked). I see no examples of company letters, but there are examples of the initials of the company name, i.e. Jasper Blues "J.B." in brass letters on the very front above the visor.

The Mississippi star (normally white) is was found in a number of photographs. This is either a metal pin or embroidered on the front of the kepi above the visor. Home made or factory made. Some made of brass, some made from the metal of disgarded tin cans, etc. The stars are worn 'cocked', one point straight down and range in size from .5 to 1.5 inch across.

The officers could wear kepi's with colorful piping around the seems of the cap. And according to the number denoted their rank.

My favorite is the forage cap, a cousin to the kepi, its crown was much more visible and was decorated much more elaborately with embroidery or brass symbols.

For a decorative kepi, to be used to represent family members, I would go with either the company initials or the Mississippi white star (not both) on the front above the visor. No color indentifying band (red was the 1st state regulation for service color for infantry, blue the second). I'll leave the overall kepi color to you. I like black, since that was the original state regulation for hat colors.

I wish I had a 40th enlisted man photo to go by.
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David Upton