Design it so that it will engage a rear bumper located at DOT-spec height (and assume it's raised a bit from the struck vehicle braking and nosing down) and transmit the impact load aft. The engagement face can be completely inside the body shell. That is, your "front bumper" doesn't have to stick out. Just sacrifice the front part of your vehicle up to that point.
You want to stop the vehicle from "submarining" completely under a car it strikes from the rear (trike has to stop before your head hits their rear bumper from underneath). You might want to consider having the structure above your legs hardened to prevent crushing, while having the impact-absorption happen above that zone, possibly by converting impact energy into lifting the struck vehicle as though the front end of the trike was a splitting maul. This is the mirror image of how the old VW Vanagons handled front impact: they had the bumper and front subframe crush back to the front axle, while the passenger compartment rode up and over the trunk/hood of the struck vehicle. So instead of riding over the struck vehicle, you're trying to force it off the ground.
Don’t forget the Spare Tire/ Windshield Washer Power as a little cushion while navigating the future trees to hug......Oh how I miss my Westy.