I never have cared for umbrellas. There's a reason Teflon and Viton positive seals are available and umbrellas are probably part of the reason. I'll give you an alternative to having the machine shop do the guide cutting. Here's the 0.530" cutter and 11/32" arbor to do them yourself. 50 bucks. Use a low-speed 1/2" drill motor with plenty of cutting oil applied by an assistant and a way to drain the excess into a pan so you can keep recycling it. Here also are Manley Viton 0.530" seals. Many manufacturers make them in both Teflon and Viton. I just happened to copy the Manley units. Measure the I.D. of the inner spring to make sure there is plenty of clearance for these 0.674" O.D. seals. There are seals available from others that are slightly smaller on the O.D.
Watch this video all the way through and you'll see why you want a little extra clearance between the seal and the spring....