Currently there are a number of locking buckles available that you could use to prevent a wearer from taking off their overalls/shortalls and other clothes that close with buckles. Especially interesting are the ones that require a magnet to open, because they look mostly like normal buckles, and without a keyhole people won't realize they are locking. On the Tips for securing strap fastenings page (at the bottom) there is more information and photo's on this.

This page is for people who are so interested in this solution that they want to adapt their own overalls that way, and like some tips on how to do that and what issues need to be resolved.

A few notes on the photos at the securing strap fastenings page: I attached the buckles to the bib a bit further towards the middle to make it harder to pull the straps off the shoulders. There are several ways to beautify it, like using cut-off ends of the shoulder straps to attach the buckles to the bib (so it is the same color and material), and removing any buttons from the bib. Note though that the holes in the buckles for the straps are fairly small, so thick denim straps might not fit though, and most overalls straps will be a bit wider than the buckles (2.5cm wide), so you get a bit of scrunching.

Of course first you need to decide which buckles to get; for the magnetic ones there are currently 2 types: the side release buckle and the one without any external features, like I used on the shortalls (see the strap fastening page for details and photo's of both). The ones I used work well (see also the Ariel 2020 shoot Trick Buckles), but I think the side release buckles might be looking slightly more like normal strap closures, and the openings for the straps are slightly wider.

Then you need to order the buckles, and assuming you want the magnetic buckles, there are a few tips on where to get them at the securing strap fastenings page.

Note that you might still need to make sure the wearer can't slide the straps off their shoulders; otherwise locking the straps is useless. See my Tips on sliding straps page for tips on that.

Once you have the buckles, these are the steps you should follow:

1. Remove the buckles from the wrist leashes/harness by undoing the stitches on the nylon straps.
2. Check if the shoulder straps from the overalls will fit through the ends of both buckle parts. This will probably require a bit of fiddling. If that doesn't work try cutting off the strap end: then you don't have the doubled up fabric of the seam. If that still doesn't work, you need to sew the buckle directly on top of the strap at the correct height.
3. Make sure the current buckles on the straps are at the correct length. Then take the overalls off and close the buckles again. Put the closed new buckles next to the current ones, and mark both straps where the top part of the new buckle should be. (Look at the pictures in the strap fastenings page to get an idea for the correct position.) You'll see that it probably will be somewhat higher up the strap than the current buckles, since the bottom of the buckle will start slightly above the top of the bib.
   If the intended wearer is available for fitting, you can also skip this step and in the last step determine the right length of the straps while they are worn. Then fix the straps with e.g. safety pins, take them off and sew the remaining part to the strap above the buckle.
4. Remove the original buckles from the overalls straps. You can also remove the buttons on the bib if you think that would look better. (Note that you probably need to destroy the buttons for that if they are the metal ones.)
5. Choose which way up you want the buckles on the bib and straps. It doesn't really matter, but intuitively I would fix the receiving part to the bib.
6. There are several options to attach that part to the bib: you can just sew it directly to the bib, or use the bit of nylon strap that came loose when removing the buckle, like I did in the photo's on my strap fastenings page. But perhaps the neatest way would be to remove a bit from an end of the shoulder strap and use that instead of the nylon. In either case you can choose to attach the fabric to the back of the bib, like in the photo's, but if you use shoulder strap material you could also attach it to the front - if you removed the button it might leave a small hole that you could cover up that way. But since it is in sight you might want to sew and hem very neatly then.
   Be careful to sew the buckle right side front: for the side release buckles the side with the circle on it should be visible.
   In my photo's I placed the bottom part of the buckle a bit towards the middle to make it harder to slide the straps off the shoulders, but if you choose another option to prevent the straps from sliding off the shoulders, like a crossbar in the back, that might not be necessary.
7. Finally put the other part on the shoulder straps and sew them at the correct height by pushing the strap through the buckle part front to back, and sew the remaining part to the strap above the buckle.