The washboard problem, if you can't get around it by multiple passes in both directions, can only be solved by basic geometry - for any hitch-mounted implement, the shorter your tractor's wheelbase and/or the longer the distance from rear axle to implement, the worse it's gonna be.
The only way I know to get around that one is to steal ideas from a REAL land plane, or build some sort of NON-3PH drag - basically just a pair of skids wide enough to support the weight of the drag, with cross pieces (blades) you can (usually) adjust for height til it works for YOU - google images for land plane, you should see a few million images (some of which might be small enough for less than a 600 horse tractor)
You already have the box blade, which in its stock form will be as frustrating for you as the rake - there are a few threads on here where people have modded their box blade with some long skids (cheaper than buying a LPGS, and done right works pretty good) - if you're interested (and especially if you want an excuse to learn to weld

) PM me your email. I can send you 108 pics of stuff like this (just the ones in my "land leveling" folder

). A lot of 'em I got right here, but couldn't tell you whose they were - plus, this site is too clunky when you try to post more than a handful... Steve