It would take 10 castings for the build not counting hardware. It sure would be heavy!
You might could simplify it some for those not too picky; like just make the bbl, cheeks and "floor" (I don't know what the bottom piece of a naval carriage is called). You could cast in the PVC pipe for the axle holes, then slip 1" steel bar stock through for the axles and use off-the-shelf cast iron wheels(?)
As far as weight goes, someone like me might not need much, especially considering shipping cost. Add some perlite to the mix.