Possible? 15mm stuff is actually really easy to scratchbuild! Even easier than in 28mm, because you don't need to do anywhere near as much detail. I use plastic for simple shapes, because it's easy to carve the 'complications' into (like that cheek-stock). Plastic glue for all the plastic-to-plastic joints, CA for the plastic-to-metal ones. Put a brand-new blade into your X-acto and get going!
I'd use a .040" (1mm) plastic tube for the barrel (maybe a .060"), and a strip of .020" (0.5mm) plastic for the stock. A small piece of electrical wire for the matchlock, and another for the match itself.
I'd see what wheels I could scrounge (I *hate* trying to scratchbuild open wheels that are only 6mm in diameter), and use more of that .5mm plastic strip to make the axle. Maybe from a Battlefront one-horse cart? Actually, using pre-made wheels, I'd use a piece of brass rod for the axle and then put a plastic strip across the top of the brass axle. I'd aim to have the wheels about a wheel-width apart.
Then put the completed gun on top of the 'axle'. Don't worry about a swivel mount at that scale, it's hidden under the gun anyway. You may want to put a bit of that .5mm strip under the wheels, depending on how you base your figures.
For crew, a kneeling ashigaru that you've cut the teppo off of for the gunner, another kneeling one for the loader, and a standing samurai in full armor to match the picture.
I'd then put all of those guys on a Flames of War medium base (I have enough of them running around and the beveled edge looks nice). 3 troops and a gun looks about right for that size base. I put the gun in place but don't glue it down, arrange the figures on the base with CA, then apply wood filler to level the cast-on bases without the gun, and put the gun back down after about 10 minutes of drying time.
It's too bad I don't have any 15mm Samurai
I could knock that out in about an hour or so, maybe less. Have to let the Wood filler dry overnight, though. Then you paint it the next day.
Presto! a heavy teppo with crew for your sieges, in about 2 hours work.
Oh, I am *so* building one of those in 28mm scale! I have enough Clan War figures to crew it, too, so