I bought T-Slot Nuts at Wal-mart

Buy Tee Nuts Online - Australia 5_8 tslot nuts T-Slot Nuts Individual Sizes ; 5971, 3667*, 4833 · 58 12 5_8 tslot nuts Standard T-Slot Nuts have an incomplete thread in in the base to eliminate any danger of screwing the stud through the nut and damaging the table

nuts and bolts slot T-slot nuts DIN 508 without thread can be adapted for special applications