To begin with let's build up both the Starboard and Port Buckets to the point where they are identical. This portion of the build will be documented in four pics.
First we want to cut two pieces of 2 x 6s so they overhang the lip of the Buckets at least 1/16" on each side. Try to not do much more than 1/8", especially on the side that the axle extender will rest on.
Using the Home Depot Buckets, as shown in the pic, my 2 x 6s were 10+1/2" long.