A video with more information about the bungee anchor is on instagram.

Finally I am ready to take orders for the bungee anchors.
Order them here: SlackX Order Form

After some more testing I decided to offer 5 standard versions.
BA1 (for 20-30m lines) (120€)
BA2 (for 30-40m lines) (120€)
BA3 (for 35-50m lines) (130€)
BA4 (for 40-50m lines) (125€)
BA5 (for 50m+ lines) (130€)

The price is always for a set of two bungees, you can also buy a single bungee or other custom made bungees, just contact me in this case. 

The length range given in the brackets is an indication for which length you have the best performance of the bungees. The aim was to simulate a 70m freestyle line, so to have a similar amplitude, similar frequency and similar "pop".
BA3 is thought to be the allrounder version, which gives you top performance on a wide range of line length. So if you don't have a specific spot where you want to rig your bungee line I recommend getting the BA3.
If you get the bungees for a permarig I would recommend to get one of the other versions.
Your weight and your bounce preference has an influence what you should choose if you are in between two versions.
I was assuming an average weight of 70kg, if you are (much) heavier you should tend to a version for longer length.
If you are lighter you should tend to a version for shorter lengths.
If you like a lot of "pop" at the highest point you should tend to a version for longer length.
If you prefer a slower bounce and more time for your tricks you should tend to a version for shorter lengths.

What happens if you use the Bungee Anchor out of the recommended length:
If you use Bungee Anchors on shorter lengths than recommended, you can reduce pretension, so you will have a lot of sag and the bottom of the bounce is getting a bit harder.
When you keep the pretension rather high you will have a lot of "pop" at the highest point which can also lead to getting airtime at the highest point. The bounce is quite fast. It's then more a mix of freestye and trickline.

If you use Bungee Anchors on longer lengths than recommended, you can leave the tension quite low, which means you have not much "pop" but therefore a big and slow bounce.
If you add more tension you risk that you will max out the bungees, then you will have suddenly a very hard bottom, which is not comfortable at all and makes freestyling basically impossible.

I hope this is enough help for you to choose the right version. Otherwise feel free to ask, I will work then on a FAQ document.
I am also trying to understand better the physics behind bouncing, and calculate the bounce behavior. I hope in the next weeks/months I can post some more insights and graphs.
Polyester, Nylon or dyneema webbing?
Thats a good question which still needs more testing. For now I would recommend Nylon, because it still gives you more bounce. But polyester has some advantages over nylon, it is more durable, often cheaper and doesn't change behavior when it gets wet. With polyester you tension it once to the preferred tension and then you don't need to do anything anymore with the rig. 
Dyneema has the same advantages of polyester, besides being quite expensive, but therefore it's really strong super light and will last for a very long time. The longer the line is which you want to rig, the more important is it to have a light set up. The problem with dyneema is that the handling is difficult, because it often doesn't work in weblocks or linegrips. Also it has basically no stretch so all the bounce needs to come from the bungee anchors. I might do some extra long bungee anchors in the future which would work then well for low stretch webbings. 

--> Test it and give me feedback.

Some general information to the bungees:
They are all a bit longer than 1m and each weights around 1kg.
I don't want to give a MBS for the Bungee Anchors, because there are still too many unknowns, especially how they are degrading and if the interactions between the different materials under cyclic loading can have negative effects. A simple pull test on a new product has very limited meaning on how strong the bungees are after longer usage.
So the Bungee Anchors must always be used with a strong backup. Industrial 1t WLL slings are perfect, otherwise I offer 6mm dyneema slings as backup. The backup should be 3m long for BA1, 2.5m long for BA2 and BA3 and 2m long for BA4 and BA5.

Here a table with more Information.

The numbers are for now only guidlines, much more testing of different version on different length is necessary.