Despite obvious, this suggestion emerge from what happened yesterday to me.

(you can jump forward, to read the suggestion down below)
In short: I was sailing when sighted a leviathan, decided to board it to mine abyssal barnacles, but there was storming the giant sea waves made my small karve to capsize. I had right the time to jump on the big creature, and my boat capsized, broke and sunk.

At that point, even being in the middle of the ocean, I thought my hopes was not lost: as usual, I have portal's elements in my inventory, and I recovered at least 10 wood logs from the broken ship, so I started by crafting a workbench... oh... wait... "invalid placement"... argh... I moved all around the giant beast, which was like a stone hill, trying to find a viable place to setup my workbench, so to craft a portal and jump back home (then, hopefully, I could have took a second karve to recover the few boxes left by the old one's sunk, or just continue my exploration sailing).

No way: no viable zone where to place a workbench. At that time I felt I was going to die, sooner or later, when the leviathan restarted sailing, so I looked up to the sky, accepting my fate and, trying to earn the best from the bad situation, I mined all the barnacles, making the big beast to submerge.
Even that didn't kill me: what killed me was the Sea serpent, called by my loud noises, when I was using my pickaxe. It chewed me to death (had no way even to use my bow, as I was in the water... if it was worthing to use it).

Quite simple (what I'm asking, not sure it's simple to implement): to allow building over a Leviathan, so that, if one remains, like me, casted away on it, but having a way to go survive (a portal, materials to build a raft, whatever) he can at least use them.

Leviathans, despite being a creature, seems mostly like an "object": you can mine part of it, you can walk on it... so why you can't even place a workbench?

Side note: whichever better suggestion, to exit by the told situation, are welcome.