Just an FYI, I have a lot of triumphs and aftermarket rocker inspection caps like the one pictured are quite popular, but just be aware there is several SIZES as well as styles of them. Preunit Triumph and Unit models IIRC are different sizes, not sure about the threads but I can measure if you like.
I have a collection of them from basket cases and projects that came thru the shop, Some of the purists dont like them at all, but I set them aside and like to use them on my own machines. But I can tell you for sure to check carefully as there is several variations in the thread sizes for openings. 500 twins are similar but slightly different than the preunit rocker boxs.
The alloy one pictured is my favorite style, and cleaned up they are quite attractive, and for extra bling you can index them and then drill small holes around the top for the Industro-steam punk look
(Match your blinged out steering damper knob similarly drilled)
There is some being reproduced today, some in steel and chrome, but not as cool in my book.
A couple other ideas since stock is not the issue here, On HDs there is some aftermarket-accy oil tank caps with a graduated taper that are oil resistant rubber and a LONG dipstick (ill leave out the puns) and situated on the top of the cap is a oil temp gauge which I admit, is rather clever and I have a few of these on my bikes (HD & otherwise).
Other options I have seen is automotive engine block freeze plugs sometimes dislodge and some stores sell a rubber plug expansion with a bolt in the center, a few turns and it squeezes the rubber into a tight fit, Bodge, but will get you home, The other is for plumping and pipe end caps with a hose clamp, and I have seen both of these repurposed on motorcycles for missing rocker caps, oil tanks, primaries and the like. A few were permanent features and dressed up with skulls or other bling. (Some quite clever)
Necessity is the Mother of Invention. (Insert Zappa reference)