may I suggest for your next trip to a bike jumble you buy something like this
https://www.ebay.co.uk/itm/389517040057?var=656840262489

I bought one , its EFFECTIVE , it works , you can keep money , phone , keys and your phone in safety , put it on , then put your jacket on and zip up , they cant see it , they cant cut it off (coz they cant see it), its albanian proof

there are many others listed on the web , ranging upto the £100s , but this one at £7.29 does the job

during the week you can put your smith and whatnot in and jumpn on car bonnets like starskey ,