The "original soaker pattern" from little turtle knits looks pretty basic. I'll bet you could do it without a pattern... and you could "customize" it to fit your baby (like the pattern says on the site) without having to spend the money.
I would knit it in one piece flat and then seam up the sides... the seam should bother baby since s/he'll have a diaper on (right?)
With a diaper on*, measure baby from side to side on the front, from waist to top of outer leg, waist to top of inner leg, crotch span/width/length, bottom of butt cheak to waist in the back, outside of butt cheak to waist , and side to side in the back. Then, you can use those measurements to knit a sort of rectangle shape (with two leg holes "cut out" on the side per measurements), seam it up, and viola! You can add some ribbing at the top or a fancy leg opening stitch, too, without having to alter your "pattern." The ribby wrap looks easy to design without a pattern, too; you'd just take slightly different measurements.
Disclaimer: I don't have kids, so have no emperical data on the ease of measuring children :-)
OOOOOOOH, and now i'm picturing "seasonal" soakers, with designs on the butt... a pumpkin, a flag, a star of david or christmas tree, a bunny face, a shamrock, etc. All of those would be easy to incorporate too, just some simple intarsia--and I think all but the flag could be done in just one contrasting color.
Maybe it could be done with words too... like "i love my mom" or "Love Child" or "Change me" or something else funny but cute. Or not... how long do you want to spend knitting something the kid's going to grow out of?? (again I don't have kids) ANYWAY, I am copyrighting this idea so i can be a billionaire some day
Anyway, if you want to go this route but aren't sure how to design the pattern, take the measurements, decide what yarn you want to use, knit up a gauge swatch, and I can put the pattern together for you.
*The diaper should be on the baby....