Adding big deck/ extra axle to a 5er. Carry 4 motorcycles.

On poor roads my fiver can get swaying almost as bad as a tag trailer when the bike is hanging off the back. Without the bike on the back it is hard to tell I'm pulling anything even on those same poor roads. Cost is the biggest obstacle I have to adding a third axle. The three axles could easily be mounted on a 2 x 6 x 1/4 wall rectangular tube sub frame and a third wheel well could be formed in the fibreglass skirting on the trailer.