Thanks Chris, I may try that in a while.
The best strap I found for stretching is a ordinary judobelt: it's soft, long enough, virtually indestructible and was designed so that you can fix it with a knot if you need and best of all, it's cheap. Here is one example for a setup of a lying groin stretch.
Fold legs and pull feet to your groin. Loop the belt around the feet and loop it the around you lower back. Then secure with a knot and place some light weight plates on your tights. I am using 2 x 2.2kg plates per leg here, but I could easily manage 20kg plates. Now slowly lower youself on your back, first on your ellbows and eventually on the back. This action will pull your legs closer to the body and now all you have to do is to relax. Try to let the legs fall wider apart with every exhalation, don't force it. When you breath in the legs will be lifted a couple of mm, when you exhale they will drop again...