Are you trying to add a DM kit to a factory stock or a chassis?
Or, if it’s a factory stock what model is your gun?
Chassis’s are baked in when made, there is no bottom metal to my knowledge; it is what it is.
But every stock that I’ve had (wood, plastic, fibreglass) will accept DM magazine kits for AICS magazines with minimal fitting, thought very few have ever been a straight drop in.
There are two options on the market I’m familiar with; Dave Waters and Atlas Works. Both are based in Australia to my knowledge but my last Atlas Works was shipped out of the UK if I recall correctly.
The DW uses a shorter 3.56 magazine and has a visible portion surrounding the magazine.
The Atlas Works uses a 3.715 magazine and is more flush with the stock. It’s also easy to find 3 round magazines at this length where the shorter mags are only available in 5 round (from what I’ve found). It can also come with an optional magazine release that’s more molded to the trigger guard if that’s preferable to the long straight release.
I don’t know of anyone making a CIP length bottom metal if that’s what you need.
And regardless of which way you go the magazines are physically quite large and protrude when inserted so there’s that.