What about an "AT-Gun package" ?
A package like a RO1 Satchel, that would be grabbed by AT-gun crew at the round start. They could then go somwhere, on even ride on a halftruck, disembark, find a nice, camuflaged place, throw the package there and it would... turn into a PAK-40 or a 45mm gun

. Tada !
It would have to be done during first X minutes of the round (to prevent players from hunting for tanks with a gun "in the pocket" for a whoole game).
Te X time could vary with the map size. On maps like Orel it could be several minutes, on small maps something like a 1 minute. After this time the unactivated packages would be useless.
After a gun is "unpacked" it could be still moved, though veeery slooowly - you could push it to make corrections to gun position or you could rotate it slowly, but it would be not suitable for moving larger distances (too slow).
Basically, you could go somwhere and put there an AT gun at a round start - ANYWHERE where you can get in allowed time. And you have an AT-gun ambush

. The tanks would have no idea to expect a gun.
It's of course less "realistic" - graphically - than an AT gun embarkable to a halftruck and towed somwhere, but still better than stationary guns (even with semi-random positions) - and really very easy to code. You don't need to separately code the embarking, towing, setting up - just code a package (taking some GFX available in the gam already) that can be thrown exactly like a sachel charge and - if the time limit is not exceeded - it spawns an AT gun where it lands. That's all

.
But to make it really usefull - the AT gun itself should be able to be pushed/moved at least a little by the crew, so they can rotate and position it carefully after it spawns from the package.