If you could find an original target card, you could simply scan and print them onto light card, so having an endless supply of them, Phil.
If you can't find an original target card, you could draw a master from Dave's dimensions and simply scan and print an endless supply off from that.
I think an aluminium horseshoe 1/4" - 5/16" thick should last long enough for occasional use --- I thought the idea is to hit the target card though.
The small privately owned museum I work for is dedicated to Harry Ferguson, so probably not of great interest to members.
Another problem is that I've never photographed anything I've ever made for them, they just bring me a photograph of something and I reproduce it.
The most interesting thing I've done for the museum is repair a completely destroyed Ferguson T20 tractor seat.
When the seat was brought to me, I pointed out that new reproduction seats were available for just £25 so why bother mending the old one --- they replied that Winston Churchill wouldn't have sat on a new one.
The seat they had was from one of Winston Churchill's Fergusons, it turns out that Harry Ferguson and Winston Churchill were good friends.
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Churchill seated with Ferguson to his right.