From my own experience from stamping mnemonic recovery words into washers I find it crucial to have a sturdy, solid, heavy and non-flexing base where the washers are placed on. So I wouldn't recommend to avoid using an anvil or some big enough solid block of scrap metal with some suitable flat surface(s). The heavier, the better...

You want to have enough mass and thus inertia that your hammer stroke energy doesn't dissipate to move or flex stuff around, except for the stamp tool letter to dig into the washer. Any flexing dissipates stroke power to wrong places.

Personally, I didn't find the need for a jig. My preference was to avoid having to buy stuff and ship it to my address that would reveal any needs for Bitcoin related storage (I don't have a 3D printer or an "anonymous" delivery address, I'd have at least to reveal my real name for any of my alternatives to my home address).

What did I use? An old hefty anvil (had it already), double sided thin sticky tape to hold the washer in place, eyeballed positioning of the letter stamp tool and a not too small comfy hammer with which I hardly can miss to hit the stamp tool, some practice to know how much energy to put into a stroke (washers are cheap, no need to avoid some practice). I couldn't care less if some letters weren't aligned perfectly.

None of my fingers were harmed, that's a plus, too. With a heavier hammer you don't need to swing it much around to put decent stroke energy into action. This helps to aim properly and hit only what needs to be hit, not your fingers but the stamp tool.