From cork to plastic
It is no big deal to make the change from cork to plastic. All it takes is the right glue and (of course) a little meticulousness with the procedure.
NOTE: Before doing the steps underneath, please check that the part is not too big for your socket. Simply insert it as it is into your socket. If it is too big to be fully inserted, a little grinding will help out, but if you don't want to do that, you can send it back and get it refunded.
If you however find it is way too small, so that it will be too loose even with the largest rubber rings, you can also get it refunded.

Here is how to boost your bocal:

Step 1
 Cut the cork off the bocal, use a sharp knife. Make sure you cut off all old glue or shellac.
This is a matter of experience, but if you feel the metal has a relatively small diameter, you may not have to remove every bit of old glue or cork. When you are as far as in the pictures to the right, try to pull the refit part onto the bocal and see if the split can be closed (as in step 3, but without the glue). If so there is no need to do more cutting

Important: if you have this type of bocal with two protrusions, you will possibly have to shorten the refit part by cutting the red section off as shown in the illustration below, so the plastic will fit between the two protrusions
For this particular Fox bocal it was not neccessary to cut off all old glue or cork.
Step 2
Mix and apply a 2-component (epoxy) glue onto the lowest 30 mm of the bocal (above the bottom protrusion)

Step 3
Gently open the refit part until you can pull it over the bottom protrusion, while placing it rotate the part to disperse the glue evenly.

Step 4
Wipe off any excess glue as best you can. It can also be removed (cut off) later when dry, but it is easier to wipe the most of it off now.

Step 5
Use clothe pins to hold the refit part closed and in position. As the bocal is smaller than the refit part, the pressure from the pins must not be too heavy as this will fix the plastic in a deform shape. The idea is to maintain a cylindrial shape and the grooves should run nicely without 'shifting' where the part is split.
Instead of clothe pins, thread or tape will also do.

Step 6
Leave to dry for a couple of days, after this remove dry excess glue if neccessary

Step 7
Mount rings as described in TECHNICAL DETAILS

  

Oh no, this is way too complicated for me, can't you do it?
Yes, I can do it for you (price: 6 euros), if you are willing to pay what it costs for me to ship back the converted bocals with a reliable service, like UPS, DHL or FedEx.
How you ship the bocals to me you decide, but I strongly recommend you choose an insured and reliable service like one of the above.
If you live near Copenhagen, it is a different matter. Please inquire.