The Wall Ship (nice name) should be small enough to Z-axis over or under with another ship--just like a planet or asteroid or anything else.
I'd expect them to be expensive, with longer construction times and have to no shields--just lots of armor with very slow passive regeneration--they would rely on repair stations to heal them.
The "guns" I envisioned are actually similar to MyFist0's jest...they would not have any real range but would do damage on contact. These aren't starbases or batteries--just large barriers with lots of very tiny automated defense guns glued all over them them. So essentially you would have to hit the wall for its "guns" to damage you.
I would make them require antimatter to move and give them a really slow recharge time.
I'd love to have them count as "civilian" structures but as we have no structure slots they would have to have some fleet cost to them. My work around is to design them as capital ships--so no one is going to spam them in place of a capital fleet.
Tech wall ships could be given an antimatter dependent mine laying ability and flak bursts--that would make them nasty.
Advent wall ships could have the strike craft magnetize and animosity abilities.
Vasari wall ships could have nanite damaging and healing effects.
I'd consider having a couple of strike craft squadrons in each and let them level like capships from combat.
In a perfect world, a Pink Floyd tune would blast out when they went into action.
In an engagement, the wall ships would soak off fire in battle from your fleets if the enemy needed to break though. An ability could be given to siege frigates that allowed them extra damage vs. wall ships and the antistructure cruisers should also be able to damage them.
The wall ship serves a different function than mines but it would help reduce them--either by replacing need for them in many cases and by making selective placement of fewer mines more effective by creating channels.
Wall ships would have to be nearly as wide as an average planet's gravity well to be useful.
...and I should get kudos for an original idea!
I've stopped updating the mods I was working with until we get past all the new updates and mines was one I was never satisfied with progress on. Frogboy stated they wanted to make mines less spammy and more practical and fun--I'm hoping so.