If you have a UKMK2 Monotype I would modify as per DDE drawings allready mentioned for conversions from MKI and new made MK2 chests.
The drawings are much too big to display here but can put them on a CD if it will help ?
You will need to make various new internal pieces so the gun is centred in the chest as opposed to being off set to the rear (hinge side) as well as other modifications.
The hole for the flash hider and centred rests can be seen in the partial drawing below.
ATB Kevin