I checked with our local "nuts & bolts" shop to find out they are getting $30 apiece for the dti's. So being proud of my Scottish heritage, I made due with two 2" long 7/16" carriage bolts and one 2" long 3/8 bolt, and a 12" long 1/2" square stock. I drilled and tapped for a 7/16" bolt one inch in from each end of the square stock( at the 1 and 11 inch marks). Then I drilled and tapped for the 3/8" bolt in the center of the stock. I screwed on a lock nut and threaded in each 7/16" bolt leaving a precise 1" sticking out of the lower side of the stock. I screwed on a lock nut and threaded in the 3/8" bolt into the center hole and left 1 1/2" sticking up out of the square stock for inserting into the spindle ( I cut off the head of the bolt, of course). The waste bolt lengths were cut off. I made sure the mounting 3/8" bolt was 90 degrees to the square stock.
To use it, I insert it into the spindle and lower the spindle until I can see both the carriage bolts touch the table simultaneously. Inexpensive and it works.
I thought of mounting a battery and an led on each carriage bolt to light when they hit the table...