For those of you that have a Sentinel and want a bag rider, shooting shed in the UK makes one.
I ordered one yesterday, I'll post pictures when it gets here.
Just got it last night. It's machined nicely, however the screw holes in the spacer for the stock where too far apart. I widened the top of the bottom screw hole in the aluminum spacer with a half round file and it fits fine now.
I should add that Shooting Shed said these are made to order, so you could probably give them your screw hole spacing and you wouldn't need to file anything like I did. You may even be able to request the spacer without the lightening cuts to add weight to the butt of the stock.