Okay, I have come up with the idea of using composite die, instead of the D20, to determine where the grenade lands. First, draw an imaginary spiral with numbers from 1 to 48 like so around the point where the player wants the grenade to land.

48, 25, 26, 27, 28, 29, 30
47, 24, 09, 10, 11, 12, 31
46, 23, 08, 01, 02, 13, 32
45, 22, 07, 00, 03, 14, 33
44, 21, 06, 05, 04, 15, 34
43, 20, 19, 18, 17, 16, 35
42, 41, 40, 39, 38, 37, 36

Then, determine who is the BOSS die, between D4 and D12. In this example, the D4 will be the BOSS die, and the D12 will be the SLAVE die.
Multiply that D4’s result to the other die’s maximum total: in this case 12.
Quick note: If the BOSS die fell on its maximum possible result, then it counts as a 0!
Now that’s done, just add the total of the SLAVE die to the result: it’ll give you a number between 1 to 48, to know where the grenade landed.

The reason why there’s a tile with 00, is because the grenade cannot land there. I always imagine grenades to deal splash damage anyway, so there are already eight tiles upon from which the damage will spill into the 00-tile!

This is inspired by this:

P.S.: you can add or remove a “ring” to simulate the grenade-throwing skill of the character. You just need to change the composite die to fit the numbers on the board.

Another detail is that, depending on the splash damage range of the grenade, if the PC is on the edge of it I might allow a saving throw to move out of the way.