Recommendation? Make boltaron sheath retain knife better.

[Sergeua]

Looking for suggestions on what to do or not do, as I haven't done this before.
When sheath came new I was pretty lucky and the fit was perfect, but now it got slightly looser. I say lucky, because I just got another bow river and it falls out right away.
I would like to shrink the sheath opening and make it safer
I'm thinking I need a c-clamp and some heat.
Please advise.

More pictures here
https://imgur.com/a/v79D2SE

Thanks,

 

[Sergeua]

So I did this for now and it works pretty good, but still would like to hear your recommendations.