Hi John:Yes, this would be a challange to build. I've never done it (although I've spent many hours playing at Putt-Putt style miniature golf courses)
I would imagine you would have two major challanges.
First, your finished surface will have to be perfectly flat and smooth.....no lumps or bumps. Trying to sink a 3 foot putt on a sloping surface would be like trying to play pool on a crooked table :-(
Second, your slab will have to be heavily reinforced and the supporting base maticulously done....this to prevent concrete cracking, heaving, etc.
You may have already thought of this, so don't know if this offers much. Just some thoughts.
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