I don't think its just a simple job as attaching a bag and inflation device to your A frame. The RNLI / Commercial RIBS that use self righting gear have specially designed and engineered A Frames to cope with the stresses put upon the A Frame when the bag inflates. If you did manage to do something on your current A frame the risk would be that it would pull out of the boat or deform the first time you used it in anger!
Self righting bags together with the associated CO2 system can be manufactured and supplied to suit say a 7 metre RIB for around £1000.The bag is normally made in Hypalon and has eyeleted strips attached to it so that the bag can be laced to the top of the A frame.Velcro fastened flaps contain the bag in a neat package until activated.The installation is quite simple,the bag can be tested by inflating with a foot pump every now and again, and the CO2 cylinder can be weighed once a year to check the fill capacity.
John.The bags normally have to be purpose built to suit the A frame. The eyeleted strips that are bonded to the underside of the frame have to be spaced correctly or they will not match the bars on the top of the frame. A simple plan view of the top of the frame showing the distances between the bars, not forgetting the diameter of the bars should suffice