You 100% sure both jump engines work?

In VB6 only first added Jump engine had any effect

Yes they work.

In the ship making screen you will have the error that the engine is too small for the ship for your military engine, IGNORE IT. It works, other ships with both types of engines can use it.

Make a commercial ship, equip it with both military and commercial jump engines.

The military one will be sized for your largest military ship, commercial for largest commercial.

Make it low tech if you want so it's cheaper to build. Range matters most as they will be stationary most of the time (You can also tug these around but I haven't played with tugging in C# yet). Attach some sensors if you want, maybe some CIWS.

Park 1 of these at every jump point. Keep in mind you only need 1 on 1 side, you don't need a ship on both sides of the jump point. This also means parking these ships in lets say Sol means safe travel to and from 1 jump from Sol and enemies without jump engines can't attack your mobile jump gate ships.

Enjoy!