HardFFA is quite different from any other close encounter map because all players start underwater.
I think that it's possible to make a close encounter map for more than two players (in 1v1, ganging up on someone is obviously impossible), but you'll have to do it a bit differently. You have to make every spawn point easy to defend, i.e. if you're on the defending side, you have to be able to kill twice as much mass as you're losing in the process of defending. This will obviously allow for turtling, but if the reward for expanding to the outer regions is rewarding as well, then turtling alone isn't going to cut it.
Make it so that turtlers can't escape very easily. Then they're not as much a threat to players who did manage to expand, so they may leave turtlers alone because they have to kill each other first.
If a player dies, everyone will want the resources. If it's a turtler that dies, then the best one to respond can build a new turtle base. Expanding players may not be able to get here because they're constantly losing units to each other, while turtlers can try to build up an army or creep to the new empty spot. If an expanding player dies, then the nearest turtler can try to expand in stead. Again: the expanding players are the ones who have to build up the pressure with mobile units and a successful turtle strategy may be able to expand because of the large unit loss.
So the problem you're going to have to solve is to make the spawn locations easy to defend (turtling is possible), but with limited space (turtling is not going to be a winning strategy), while also being easy to break out of in the case of someone's death (Changing your strategy is an option).
Good luck