Well, I would take one of the mounts that Thule makes to attach their bike rack to a rear mounted spare tire and use it as a base to mount a "rod holder" to. Item "B" in this illustration is the part I mean... It's $20 from Thule (p/n 753-2075-02)


I'd buy that part then fab the part that holds your rods. I'd set it up so it will screws onto that mount since IIRC it already has a "convenient" screw hole.

In case it's tough to understand what I mean, here's what I did with a similar mount but instead of a holder for fishing rods, I used it to hold up my HiLift. You can see the bolt head of the 1/2" bolt I used in the HiLift.