Like I said, find the thickest wood or metal drill bit, and drill a hole in the middle from the top to almost the bottom. It will work for most people initially. If you can't insert, wobble the drill bit at the top a little, or just find a thicker drill bit.
The diameter and depth of the hole is not important, but from the 2nd time you will want to custom for your own needs.
It does not matter that much if you drill all the way to the bottom. Typically the hole will be a lot smaller than your little head, so you will find a lot of cushion when you are reaching the bottom. Even hitting the flat bottom doesn't hurt because your dick is soft. If you are really long or you are worried, you can stack two cans end to end. It's a metal can so you can find a lot of ways to hold them together - weld them, screw them or clamp them.
The diameter of the hole should be the smallest that you can still insert. It will be different when you are not fully erect and when you are. You can always have two or three at hand if you need it. One is a lot better than your hands anyway. And it's always custom size as compared to Tenga or the Fleshlight.
For normal drill bit sets, for wood and metal, the thickest size will be good for many people. Any thicker you need to find specialty drills, to get 1/2 inch holes or larger.
Alternatively you can take the block out, cut it in half, carve something in the middle, and put it back into the can.