Make your own. Slim line pen kits work fine. Turn your lower barrel out of antler. For the upper barrel you will need an aluminum arrow shaft. One arrow shaft will make several upper barrels. The size of the arrow shaft will/can be an issue but is easily over come. The shaft most likely will not fit your brass barrel from the slimline pen kit. Easily over come by turning a wood insert to fit inside of the arrow shaft. (Brass pen barrel will be inserted inside the wood insert.) When you buy your aluminum arrow shafts, you can buy the brand that your customer hunts with. Then use the name that will be stamped on the shaft to make it really customized for your customer.
Check out the library here at IAP, I believe there are some tutorials dealing with the above design.