I have a low budget project underway building a 500 S&W on a Siamese Mauser. For the record, the thing dang near feeds dummies perfectly, hence the motivation to move forward with the project. The next hurdle is sourcing a stock. I'd love to find some kind of drop in synthetic but it looks like that is asking asking a bit too much. I can deal with a semi inlet if that is what is available. Any suggestions.....?

Only game in town; Richards. No one makes a drop in synthetic; there is zero demand for them. Ok, one per year.
They do not have the Siamese in their CNC 99% inlets; only the old pantograph type. So hand inletting is required.
PLT for them is about 8 weeks. On the bright side, their wood is better than anyone else's, dollar for figure.
What barrel did you use?

Tom is right on there about Richards. I ordered my stock way back in March if I remember correctly. They don’t typically keep them in stock and I believe they wait to see if they get more orders before starting up the pantograph machine. My stock finally showed up the end of June. The outside was close to final trim, but the inside needed quite a bit of inletting. Overall, I am pleased with it. Just be patient .

Using Douglas barrels. I haven't done any feed work yet but they pretty much feed as is. Needs some bolt face and extractor work to get the cases up in the bolt face and a block in the mag box but other than that it appears it will be a relatively simple conversion (knock on wood here!). The follower isn't ideal I will either replace it or add a little metal to move it over to the right a bit. Stacked they feed perfect. It will have open sights only, I will alter the bolt to an original sporter configuration and retain the safety as is.

I do like the Siamese mauser and built the first one in 1974; using a Richards stock. New Siamese rifles were $19.
I removed the barrel in the supply room of the company I was XO in, at Ft Knox. The Supply sergeant and a Platoon sergeant who watched me thought I was crazy.
Sometimes even a standard 98 follower works better.
Extractor; bevel the front face at the bottom.

This is my synthetic British style stock that I have made for me from my pattern. It's designed for a drop box. It's designed for my Satterlee Mausers but I've retrofitted FN actions to them which requires some extra Marine Tex. We've done them in both fiberglass and carbon fiber.
It may be a workable solution for your project.