I fitted a Dr Bobs quick fill to my rapid. You remove the valve from the bottle and fit the quick fill assembly to the gun where the bottle used to screw in, then screw the bottle into the new filler/gauge assembly. I needed to cut a hole in the stock for the new inlet as it’s an in-line fill and gauge. V configuration are available as well for the rapids, not sure about bsa’s? Simple enough job Ben. If you get one and you aren’t confident fitting it let me know. ??
just seen the link, even easier than I thought then?