You don't mentioned how your 500 is to be used....
I'm a serious handgun hunter and have been for over three decades. I hunt some very treacherous country (high rim rocks, very steep ridges and very rugged terrain where others don't commonly tread..
I've found in those conditions a chest carried holster doesn't serve me well because I spend about half my time crawling on my belly or climbing a steep cliff. A traditional shoulder holster that places my 6.5" 500 under my left arm has been more accessible and more protected under those conditions.
Prior to buying the 500 I use 29s and 57s with 8 3/8" barrels. I used both Bianchi and Safariland shoulder holsters for them. A 10.5 500 is quite a hunk of metal to be lugging around in any kind of holster.
I believe I'd look into a clam-shell type spring loaded standard shoulder holster that functions like a reverse break front.
As you seem aware, your 10.5" is a special problem because of its size. I chose a pouch type for my 6.5 because it can be drawn with a short pull. What ever you chose I'd recommend a break open design.
The company I chose, the A. E. Nelson Leather Co, makes 500 holsters to order. They don't list the 500 on their site but they will make them any way you order them. They have been in the law enforcement leather business for decades. Their web address is:
A. E. Nelson Leather Co.
The design I'd suggest is the "Untouchables" model at the address below on their website...
A. E. Nelson Leather Co.
Below is a picture of my pouch type which is not a break open. Not the fanciest but well priced at $80.
