The long bayonet is a nice correct touch. WWI trench guns were at a disadvantage in bayonet fighting, because the gun is short,even with the long bayonet, compared to the rifles of the enemy and their bayonets. Still a handy thing to have at need.
I'm not certain, but I believe that even when we went to the shorter bayonet for our rifles, we kept the long bayonet as the correct issue to go with the shotgun.
A lot of people questioned the long bayonet, "why use one twice as long as the enemy is thick??"
Until they learned that when you use a rifle with bayonet, REACH matters.
Going to the shorter blade during WWII was done because it was recognized that while still important enough to train troops to do, bayonet to bayonet fighting was on the decrease, compared to WWI.
The trend has continued to this day, bayonets are useful as knives, and for herding civilians, much, much more than for bayonet vs. bayonet fighting today.
Maybe the Marines still train some with it, I don't know. I do know that when I went through Army basic training (in the mid 70s) our bayonet training consisted of a DI holding up a bayonet, "This is the M7 Bayonet!!", then tossing it to stick in the ground at his feet, and saying, "You will not use it!!!"
If you want to add a little more "correct" look to your trench gun, I know a guy who got one of the Ordnance Corp's "Flaming bomb" stamps, I believe you can still buy them. As long as you don't try and pass off the gun as authentic original, its not fraud, its a "reproduction".