Profoto make a case with a shoulder strap for their A1 flash. I'm sure there are similar cheap knockoffs somewhere.

Godox makes a similar type bag for their much larger AD600 Pro.

Many lens cases come with shoulder straps, Some might be the right size for your flashes.

I just use the cases that come with my Yongnuo and Canon speedlights. They have belt loops on the back of the case, and it is pretty easy to holster/unholster them when they are attached to my belt (either the one holding up my pants or a separate camera belt for holding lens and accessory cases). You could probably just run the ends of a conventional camera strap through the belt loop and connect the two 3/8" ends through a single one of the loop buckles that come on each end of the strap.

It's not exactly a strap attachment, but there may be another way to get where you're trying to go.

Spider makes a product called the Spider Monkey for attaching speedlights and other accessories directly to a belt. These look to be not much more than an old fashioned "universal" phone swivel belt clip with an adhesive tab that sticks to the back of a phone.

Stick the tab to your flash instead, put the clip on your belt, and you're done.

An earlier version of the Spider Monkey used a velcro strap to hold the tab to the flash.

Take a wide flash accessory strap with velcro connectors on each end, cut an appropriately sized vertical slit in it, and use the tab with or without the adhesive as an additional way to secure the flash.