If you have access to a lathe and a mill, and are willing
to make shortcuts, making one wouldn't be that hard. It
looks like the sight on ebay actually uses a Lyman slide.
You can save yourself a lot of time by starting with
a bolt shroud and modifying that. I tried to make something
for a Mauser. I got a lot of it done, then life (actually twins)
and other projects interfered. Here is what the Mark I version
looks like
I think if I come back to it, I will put a Redfield slide on it.
If you have any questions, send me a PM