My uncle managed to do it with mole grips instead of the supplied T bar and the bolts grip well. Job completed

Ok here we go. As a precision engineer, I drill and tap holes of all kinds regulary. Which also means the odd troubleshoot. The correct drill size for an M8 thread is 6.8mm however as we are not after something that will hold up an RSJ you will be fine using a 6.9mm drill which will no doubt come out at 7mm drilling it by hand. Next get the tap started. Add a little lube if you like. Then slowly turn the tap. This can even be done with a small adjustable spanner with care. Every 1/4 to 1/2 turn turn it back a touch, to break the swarf. Take your time and keep it straight and you'll breeze it. If you mess it up don't worry. Something can always be done at little cost. Go for it!

ok a bit late from me sorry. nice job