Guys;
aspects to consider.
keep it small, at the biggest two clover milk crates is big enough.
if you build it bigger, it ways more, you load more crap into it, and then you suffer.
wheels:
we have found that bicycle wheels 20 inch works fine.
thin wheels do tend to cut into the sand where the sand is harder. they then tend to move better over the sand then the wider wheels, as the wider wheels have more sand and friction.
try to get wheels with plastic rims as they dont rust.
also get wheel with the 10mm shaft as the 8mm shaft tends to bend under heavy loads.
we have also found, that making the cart to be pushed by a rod stand/ pension pipe helps with the human motion and a woble effect is given to your hands.
with your hands / arms absorbing this back force of the trolly, it is pushed much easier over the loose sand.
currently i have made mine out of steel which is heavier then aluminium. one day i'll make one of aluminium.
rods can be upright, as long as your spacing is so that the rod tips dont bump against each other.
when rods are lying on the cart, see how high you can get them, as the sand have always gotten a way to get to the reels.
jsut my 2 sents
ss