TDC Locator Tool

Take an old spark plug, break off the insulator and drill a hole down the middle. Push a rod (ideally plastic or wood) down though and away you go. Not particularly accurate unless you can set a dial gauge to the end. With a dial gauge it's best to find the points about 5 degrees either side of TDC where you get a useful displacement rather then go for TDC directly.

Do make absolutely sure that you're near TDC on the compression stroke when you stick something down the plug hole.

This method can be absolutely dead on accurate if done properly.

Do as Stan says but also drill and tap to the plug body for a grub screw.

Put it in the plug hole and set the rod so that it contacts the piston.

Fit your degree disc to the crankshaft  and set up a pointer.

Set your degree disc to say 45⁰ btdc then turn the engine backwards until it touches again. Note the reading atdc then adjust the degree disc by half the difference. Rotate the engine again and note the reading btdc and adjust again. Carry on until the reading is exactly the same after and before tdc, now your degree disc is set perfectly.

Remove the rod, then turn the engine to the  point at which you wish to set the ignition, re-insert the rod with it contacting the piston, and lock up the grub screw, now you have a setting gauge for setting your degree disc and points, bang on.

A precautionary word: if you have high compression pistons with a pronounced crown, and not LC flat ones, this might affect your readings.

Naill's method removes any discrepancy from having funny shaped pistons or spark plug angles as you are find the exact same point from two different directions.