I drilled a hole in the end of the lumber:
Here is an example of the 3/8" connector:
which I then hammer into the hole:
all the way in:
Then I thread in some 3/8" inch hooks left over from some turnbuckles. One could also use 3/8" bolts too.
The installed leg:
I expect the rubber mat to compress with time. I will then unthread the hook to take up the slack.