Seal for pneumatic air tamper

I am currently working on using an air tamper to cut down on some manual labour but need some advice on seals. What I am doing is using a chinese made air tamper to ram up posts to save doing it by hand which is quite labour intensive.

Being of cheap manufacture the current one uses leather packing as a seal to keep dirt out as it is used in the hole the reciprocating shaft gets dirt on it and so on.

Wanting to install a cheap wiper seal to keep the innards free of dirt. It runs at around 5-10 beats a second with about a 2 to 3" stroke. It does not matter if the seal only lasts a few days provided it is cheap say a few dollars as it could be replaced in five minutes.

Any ideas on a seal type in 14mm ID? I can get U cup seals for $3 each. It is the shaft speed that is the thing.

MMMM? this is me thinking aloud so please take it as such working on top speeds, 10 beats a second is 600 / minute, and with a stroke of 3'' that's 200ft / min which is not exactly fast, …..so I'd go for a say a neoprene etc etc type seal and assemble it with silicon grease.