Although I could be wrong, I do not think anyone offers one.

May I suggest replacing that Ruger with a Reimer Johannsen with a Rigby floorplate?

I almost hesitate to comment here as I don't have a "500" in my screen name, but here goes.

The only aftermarket floor plates I have seen for Ruger bolt-action rifles were steel units to replace the aluminum units on the M77 Mark I (tang safety) models.

jim

I had to change the magazine spring on my Rigby but it (barely) holds 4 down. I have another 550 built by Lon Paul in .458 Lott that holds 6 down and one up the spout. If you had to wade into a herd of big, less than friendly, critters it would be the one to carry.