Summary

I can't stress enough that this is in the very early stages of development

Lost your spring cap because you forgot your blaster was primed and went to change the spring or the cap worked its way out?

There sadly doesn't seem to be an option to buy a replacement spring cap at the moment, so this is intended to act as a replacement. So far it has not been tested during play but seems to hold up

So far it's held up with a 1.6 spring but hasn't been completely tested during play.
The original version was intended to utilise the allen key provided with the blaster but further testing showed a tendency for this to strip with normal use, so there is a second version which allows for an M5 bolt, just glue the bolt in.

just be careful and assume that the part will likely break with little notice

Addit: this evidently might only work on the original white swift. I've been told that the later coloured versions have a different pitch