I don't know if this would work, but for carbon-fiber bikes, they make a product called "carbon fiber assembly compound" or "carbon friction paste". It is a grease-like material with microscopic beads of glass or sand or something mixed up in it. It is used to keep slippery materials from sliding. Normally, I think it is applied to a seat post, which is then clamped down in the seat tube clamp. The extra friction of the paste keeps the seat post from being able to slide down. Anyway, I have no idea if it would work and even if it did work, I have no idea how long it would work. But I just wanted to let you know the products exists if you want to try it.

Heck, I have some which I could squirt in a small plastic bottle and send your way if you wanted to try it. I bought it but haven't ended up using it for anything. PM me if interested.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).