The base fits in the bottom of the barrel to carry the weight of the person inside without bending the barrel, chassis or axel. It's made by screwing two bits of 2x4 to some ply wood. We cut the edges of the ply wood off so it fitted neatly in the bottom of the barrel and arranged the 2x4 so it wouldn't lift up the base. We then screwed the 2x4 to the ply wood and drilled 2 holes in each 2x4 to make a hole for the axel.
Of note is Kitty in this picture. She is happy because dogs don't get to ride in Waterslideboxcars. Wimp.
Drill axel holes in the barrel
We marked out the right place for the axel holes in the barrel and then drilled them really big to allow for discrepencies.
Attach base to barrel
We then bolted the barrel to the base in 3 places each side. It seems pretty secure but I'm sure we'll put this to the test.