Sounds to me like your talking more about the rod wiper than an actual rod seal. Most rod seals are of a relatively high durometer but not as stiff as an actual plastic like your describing. The wipers, on the other hand, are typically made of alot tougher material than the actual seals, and that, coupled with the shape is not condusive with the use of the tool DH suggests. That said I don't know of anyone making a tool to help insert a seal/wiper/etc made out of that hard of a material.
Now if it's actually a U seal of some description then the instillation tool DH recommends can't be beat. I spend alot of years putting seals in with no tool, and when those came out I would have given a mint for one as they make putting a seal into it's hole alot easier, especially when it's a small diameter seal going down into a hole your fingers won't fit in.
With all of that said, the harder seals will typcally benefit from being soaked in HOT water to soften them up a bit before putting them in. Too, there are small diameter buffer seals (don't figure that's what you have as they typically have an oring energizer in the groove behind them)that need to be prestretched before installing it in the bore. Failure to do so on those will typically end with the seal being messed up when the rod is insterted through it.
Hope this helps and good luck.