The inventor of the Penis Plus deserves congratulations on a clever design. Alas, the price tag of DM 298 (~$US 160) invites you to make your own at a fraction of the cost. (The original is at www.penisplus.com.)
The concept is simple - pull your penis to the side with a belt running around your waist, anchored to the base of the penis.
All you need is an ace bandage. Tie a simple knot to secure the ace bandage to your penis, run the bandage around your waist and loop around base of penis. Tie it off using a taut line hitch (see below) and tighten as needed.
After you do it once, you'll see why some extra padding is a good idea. So, before tying the end of your penis again, wrap a piece of cloth around the penis a few times (I use a short piece of the Ace bandage).
To make this work, you need to know the taut line hitch - a knot that will serve you in many other ways (great for tying down loads on cars, for example). This is how you tie it using the swim cap:
Here's another diagram (thank you, Boy Scouts!):