Converting outlets in 3 3/4 boxes to GFCI
If the box is not deep enough, as sometimes happen, get a "box extension". If it's a finished room with the boxes embedded in the wall, try a "surface conduit starter box".



If it's simple, industrial style surface mount metal box, you want either a "4x4 box extension" and/or a 2-gang mud ring, as needed to provide depth.   Then you use any common Decora faceplate. Do not use dome faceplates, unless absolutely necessary, because this. 







There is no earthly reason you should need to shop for these online. Most home improvement stores will have most or all of this; a local electrical supply will be able to hand it to you in 30 seconds (and be cheaper lol). 

Remember you only need one GFCI device per circuit.  The GFCI protection can be passed on to other plain outlets, if they are connected to the GFCI's Load terminals. 

If you do not intend to avail yourself of this feature, do not use the Load terminals at all - leave the "For Wizards Only" warning tape on them.  Load terminals are not for onward connections (unless you mean to protect them obviously). 

And especially, do not waste GFCIs by connecting the Line inputs of one GFCI to the Load outputs of another. That is sheer silliness and will create real complications when you try to reset them after a trip.