I've got the goose neck, which flexes more than the straight neck, and my dozer of about 18 pounds barely flexes the trailer bed.
I'm using long M4 bolts to connect the parts, it gives it alot of strenght.
My guess is you'd need a better suspension swing with bearings and better wheel hubs before having to upgrade the bed.
I will probably mod something to that effect later, as bolts through PLA are hardly long lasting as wheel shafts, i'll need something better along the road for sure !
Click (Post a Make) and upload some pictures if you can/want :D