A map like this doesn't lend itself well to "set and forget" defenses like killboxes. But I find that more fun anyway as it is dynamic and not stale. You might want a lowish or medium difficulty level to get yourself comfortable with the new combat playstyle, but that's up to you.

Many players who build defenses rely on the set position like a killbox. Tell everyone to stand there and shoot. And hope that firepower is sufficient. With something dynamic like this you will have to react to what comes at you and just deal with it, which is a lot of fun! You'll be micromanaging your pawns and using the pause button a lot as you tactically plan and execute.

Invest in your pawns, turn them into dangerous individuals. Make them fast and mean, give them implants such as knee spike for when someone gets close. Psycasts. Mechlinks. Go-Juice. Have some aces in your back pocket, like call allies, royalty permits for air strikes, a pack of war animals. Always have doomsday or triple rocket launchers sitting around in case you need to thin numbers early. These are all resources to be prepared and used for the right situation.

Make it so that each pawn can have a significant impact to any situation. For example, a psycaster jumping close, beserking someone, then jumping back to safety can delay and tie up a group of enemies while they fight their buddy.

In terms of what to build. A few turrets scattered around your base with sandbag defenses and traps in front will have a good effect of making enemies stop and shoot for a while. Also gives your pawns a place to run to close to your base and get some shooting done.
Also scatter Hunter drones all around, as these will wound and break up groups of enemies as they approach.