You can get the outer fork tube off without having to remove the cartridge.
A stick, ruler, whatever is fine as long as you can accurately see where it gets wet.
A PVC pipe might not have enough heft to slam the oil seal home. You need to make sure it's exactly the correct diameter. The easiest way to get a seal in is to heat the fork tube up with a propane torch for 30 seconds or so, then the seal should press in just about barehanded. Help from a slightly smaller-diameter washer or PVC pipe should get the job done.
Thanks for the advice. I just ordered everything to do the job. I am thinking of trying to get it in with electrical tape as shown on a youtube video and might try to do the heatup method as well. I went to a hardware store and bought a bracket I can screw to the garage wall and will use to 5/16" bolts and a floor jack to compress the spring. I did try to clean out the seal with a plastic tool but still leaks a bit. Thanks for the ideas and advice.