So I am wondering, would anybody be able to recommend a type of ear/flesh tunnel I can buy to help with stretching? I can currently stick in two thumbs and stretch the hole comfortably, but I cannot yet retract it all the way while flaccid, so I am thinking a couple ranging from 6mm to 10mm, but any idea on the type? Any thing I should look out for?

Can you find them locally so that you may examine them? If so, I recommend that you gamble on style by purchasing a couple or three to try them. From my perspective, one which resembles a spool on which sewing thread would be the best choice, because the little lip would encourage the end of the skin to stay in place on the barrel.

We seem to have a problem here in description. Are you saying that you can fit both thumbs together completely through the opening in your foreskin at least as far as the last joint of the thumb, so that they are then sitting alongside your glans? Two thumbs together are at least as big as your glans when flaccid (soft), more like 20 or 30 mm diameter so if this is the case it should be fitting easily through and inability to "fully" retract when flaccid would be due to the frænulum which then needs to be stretched as indeed, it almost always does if you have not been retracting (or able to) the foreskin.

Please explain further if (where) I have misunderstood your description.

Almost all "flesh tunnels" have a rim or "lip" which holds them in place, though some achieve this by having a groove in which an "O-ring" will sit. This makes them longer which is in itself a fashion statement but quite inappropriate for our purposes as you do not want them to press against the glans and therefore want them to be quite short - the tight region of the foreskin is generally itself fairly short.

There is always a risk of sensitisation with metals other than silver or (pure) gold, nickel is very bad in this regard and is used in the plating process of cheap jewellery. Plastic such as acrylic or silicone is very unlikely to be a problem, and since you are (presumably) not concerned about appearance and do not expect to be using any particular size for any significant duration, you obviously want it to be cheap!