Looking for some suggestions on a good way to secure my battery box and fuel tank so it won't slide forward should I need to stand on the brakes while trailering.

I'm not really coming up with any good ideas, and I don't want to bubba it, which I occasionally have been known to do.  

Any ideas would be great!

Marine battery tray with bolt down top (~$15 at West Marine or Amazon) for the battery
put a hinged cover/door over the whole compartment to keep the fuel tank in place.  hold it shut with a turning cam lock.  if you want it to look less "bubba" get a spare cut of the same marine carpeting to cover the door (might check shops for scrap pieces since you need a small piece).
if the tank it slides side to side, screw a bump board/angle iron into the deck to keep it from sliding all around.

It's not quite the same problem that you're trying to solve, but in my old 23-foot sailboat's built-in cockpit storage lockers I had a wooden frame that was attached to the hull to which I used an X-shaped bungee/shock-cord arrangement with a hook at each end to hold down the gas tank - the center of the X went through the gas tank top handle, and the hooks at the ends of the tensioned cords were held in place in metal eyebolts attached to the wooden frame.

As for the battery, it was kept in a plastic battery box, and there was a strap that held the battery box down - the strap went through a hold-down bracket that was fiberglassed to the hull.

With the sailboat, the main concern was preventing movement of the battery or gas tank as the boat heeled, rather than the 'abrupt stop when braking the hauling vehicle' problem that you're trying to handle - but it's possible that the same concepts may be applicable to your case.

As others have said there are straps you can put on the battery and fuel tank.
You screw the little bases onto the floor.

A piece of 1 1/2” aluminum angle screwed down in front of the battery and fuel tank would help. If the bench is removable you could bungee from the angle to the back of the boat for a little more security. But personally I’d build a door and hydro turf it close to the color of the carpet.

Some great ideas here.  This is why I love this place!  

Yes, room is quite tight and the top piece/bench is not removable without removing the screws hold it down.  I like the idea of the angle iron/alum if it still gives me clearance to pull out the battery and the gas tank.  I think that is the main concern.

As noted the battery does not have cover.  I bought it this way and I too suspect that there wasn't enough room for it.  I've actually mounted a battery up front now for the trolling motor and electrical needs (lights, pumps, fish finder).  So I will likely replace the battery in back with a smaller battery dedicated for just starting the motor.

I secured my 13 gal aluminum fuel tank with stainless straps I fabbed from a Breeze kit.https://www.caphardwaresupply.com/products/breeze-4000-make-a-clamp-kit I fashioned quarter inch thick rubber strips under the straps to prevent chafing.

My group 31 and 27 batteries up under the front casting area are secured in custom snug fitting wood surrounds with heavy duty solid rubber bungee straps and stainless ring bolts to hold them down.