I would *strongly* advise staying away from undercoating. It does just fine at letting rocks & pebbles bounce off of it but it has little to no abrasion resistance. Everywhere that you place your feet or put cargo it's going to get all balled up, smeared around and transferred to your shoes, pants, cargo, your dog, everything. Come summertime it's going to be a REAL nightmare especially if you leave the cap on the truck, it will make a mess the likes of which you've never seen before, might as well just paint it with gear oil.
A roll of inexpensive indoor/outdoor carpet works well enuf and I've known a couple guys who did a great job using cheap artificial grass, plus the astroturf is easy to hose off *especially* if you use neoprene seat covers.