Least expensive Fat tire tadpole I can find Is the Trident terrain and with the 20 inch tires and the 3 chain rings I can get the 13.5 ish gear Inches I want to climb my gravel roads.
would also love to see a Home build.
that is basically what I am working on. Have the 26 x 4 inch rims and cobbled the two front hubs together to fit the wider rims (a 2.5 inch wide hub cut in two, added a spacer) have pics but need to upload. search for RECON LIZARD. It has a link to a factory made trike similar to the one I am designing.
My next project after the SWB two wheeler will be a fat tadpole. Mine will be definitely NOT be an off roader though. I'm using moped front (16.5.") and superbike (24") rear wheels and tyres and if I get it right it'll be belly dragging low. Those wheels will add a lot of unwanted weight but could be swapped in future for cycle based fatties if funds allow.