I don't know what material your original seal was made from, but I've made a couple piston seals from 90A-95A polyurethane (PUR) rod like thishttps://www.mcmaster.com/9697K138-9697K69/
for sub 20 ft-lb rifles. They shot a little under factory spec and only have a few hundred pellets through them, so I can't say how long the custom seals will last. I ordered new factory and aftermarket seals for those rifles but the restock and shipping delay at that time was like 2 months and I just couldn't wait. I NOW have the new seals but my PUR custom seals are still working so I plan to keep shooting with them until performance drops or there is a seal failure.
I initially tried cutting the PUR with sharp tools on a wood lathe, but that just didn't work with this relatively soft "gummy" material.
I used a small band saw to cut a "disc" off the PUR rod. The disc was a little thicker than the final seal thickness.
Then I used a bolt, washers, and 2 nuts to "chuck" the PUR disc for spinning in an electric hand drill and used my little 1" belt sander to carefully shape the OD. Worked SLOWLY and got a really nice fit into my compression tubes. To get the correct inside taper ID was a little tricky. I turned a wood dowel to the correct conical taper and then glued sandpaper to it. I chucked that up in my drill press and slowly CAREFULLY introduced the seal to the tapered sanding cone. The tricky part was to stop and rotate the seal often and remove material s-l-o-w-l-y. That helped make the taper even and centered. I CHECKED dimensions often AND STOPPED before the hole got too large.
The PUR rod I ordered from Amazon (no longer available) was 90A hardness and cost about $2.00/inch. Each seal only needs about 1/2" so material cost for mine was ~$1.00/seal. With shipping I estimate around $2.00/seal for the material linked above.
High density polyethylene "might" work. Teflon can cold flow and not return to its original shape, especially if it is hammered by a steel piston. I don't think Teflon will work as a parachute seal in a spring piston air rifle. There are likely other polymers out there that might work, but that's all the info I have.