I feel your pain. No magic solution to offer other than what you're probaby already doing.
The simplest technique is to drag the boat on shore, tilt it on it's side, install cart & tilt back on wheels (this puts a lot of stress on the drivewell BTW). This doesn't work for me.
I have a bad leg & back so I avoid lifting the boat as much as possible. I also fish so my boat has a lot of gear that doesn't allow it to be tilted on the side unless I can unload it first (rare occurance). I want the cart installed in the water so I can wheel the boat right out.
I get the boat about knee shallow. Grab the cart crossbar with one hand, put the other hand through the mirage drive well. Slide the cart sideways under the boat until you can grab the crossbar through the drivewell. With the now free hand, I place my thumb in one of the "sockets" in the drivewell & grab the corresponding cart "strut" with my fingers. Guide fingers/strut to thumb/socket & start tilting cart down until strut just starts to seat, then swing the cart so that the other strut can also start to seat in the socket. Tilt down cart to complete. I find this easier than trying to get both struts lined up at the same time, but to each his own. I also have ape-like hands so that probably helps