Check out the contoured dove tail offering from Ranger Point Precision. Its around 30 bucks.

Rifle Barrel 3/8 Dovetail Filler Blanks | Black Nitride Steel, Stainless Steel Options.

Marbles used to make one for Octagon barrels.

I think they offer the best aftermarket levers for Marlin and Winchester, slightly bigger, but not goofy looking like some.

Their aftermarket loading gate for Marlin lever guns is a must have for me.

Good shooting Pard.