Large rafts spawn from the start of the game, and thanks to a recent base game patch, they spawn Spitters/Stompers immediately. This makes sense if you're coming from another planet and have modular armor and weapons and damage upgrades, not so much when you're 5 minutes in and have 8 stone furnaces to your name. Your only chance is to look for a seed that doesn't have any large nests too close and hope they don't form one too soon.

Rafts are considered a resource (reasonable) so the starting area setting apparently defines an area where they spawn by the lowest frequency/size setting, rather than blocking them entirely like on Nauvis.

Some kind of grace period is really needed now.

I'm doing it rn, it's still possible but it's a pain. Dealing with large raft is just impossible without rockets.
You are defenseless until you research rocket turrets, which is past blue science and also requires a LOT of coal for the black science.

But you have to limit the amount of crop you gather otherwise an attack and it's almost game over. I mean, with base upgrades you need 15 turrets with red ammo to kill a single small stomper. You have to manage your cloud of spores so it doesn't reach the first large rafts.

With your trickle of resources it would take days to gather enough iron and copper so 10 hours into the game I was still harvesting rocks.

You ran out of seeds very quickly in the early game cause you just have the jellynuts to burn. It's a pain until rocket fuel.

The coal production is abysmal, you need 5 carbon for a single coal and 10 coal for a single grenade. Black science took hours even with dedicated spoil generation.

It took me about 15 hours of suffering to get the rocket turrets.

You've been warned