I tried a third handcart hike not only to go farther that I had on the first, but also to test design improvements I had thought of during the second hike.
The changes I made to the handcart are:

1. Replaced the thick dowels (end portions of the long handles) with
1x3 boards to facilitate a better attachment of the crosspiece
2. Attached the crosspiece using screws to make assembly easier
3. Added vertical dowels to the handles for better gripping
4. Added a “kickstand” to elevate the cart’s handles during rest breaks so I would not have to bend down to "pick up" the handles
5. Added an extension to the front end to provide more storage space.

As with the second handcart hike, I arrived at Crater Island in the evening and inspected the mud flat to determine whether hiking were feasible. The mud was just barely dry enough, so the next day I took a chance and began the hike. I hoped to go seven or eight miles, passing the place I camped on the second hike a year earlier. Going that far would allow me to reach the goal of these hikes---the three small hills at the northeast end of Crater Island.

The cart modifications made using the cart much nicer, which lifted my spirits. The condition of the mud was tolerable for about two miles, keeping my spirits cautiously high. But then my worst fears were realized---soft, wet patches of mud began to appear and quickly morphed into a vast expanse of slippery, sticky mud that made travel impossible.
I gave up and retreated to the car, demoralized.

The three hills still call.