Good advice on keeping the fork tubes clean. I've been wiping them down after each ride but with virtually no winter weather here this year the bugs are thick. Whatever I pulled or pushed out with the Seal Mate must have been really tiny because I never did see anything on the tool, maybe it got pushed past the seal and into the fork, I don't know. I never even had a drip, just an oily ring so it wasn't much of a leak.
I have read of people using 35mm film or other thin plastic. My concern would be tearing or breaking the home made tool while it's in the seal and having a piece get stuck below the seal level. I suppose that could happen with the Seal Mate too, but it's pretty flexible and still stiff enough to do the job.
I hadn't thought of using a feeler gauge. As long as you can be careful not to damage the seal I would think that would work fine, though it's smaller than the Seal Mate and could be tough to hold onto while you work it around the fork. I try to always remember the first rule of home maintenance. First, do no harm.