I believe
@tmassey had tried a similar bag approach and had a scary story of coming out of the water to discover many urgent voicemails and texts from the coast guard because the pressure on the PLB in a bag had caused buttons to be pushed. He called back just in time to prevent the helicopter from taking off ..
He talks about it in his early review of the XL canister .. which I'll take the opportunity to blatantly promote again with this link
https://scubaboard.com/community/threads/dryfob-xl-affordable-canister-for-plb
Of course there are other options available for cannisters also, and this thread provides lots of insight into mounting and using a cannister and would be applicable to most if not all cannisters you might choose.
We've also added mounting/attachment tips and device size/fit lists on our website should you choose to use a DRYFOB.
FAQ - DRYFOB Many would also apply, regardless of the cannister you choose.