Esstac sells their KYWIs with industrial grade Velcro attached to the sides and the bottom of the insert.  Esstac also provides a matching industrial grade sticlyback strip presumably for installation inside the pouch where it will stick to the mating Velcro already on the kydex insert.  Some vendors to not include the extra Velcro, so ask about it before ordering.  Of course, the inside of the pouch needs to be clean for the very sticky industrial grade Velcro to adhere properly.  Industrial grade Velcro, which is very durable, and super-sticky, is easily available on Amazon, etc.. 

Being able to turn the pouch inside-out is a huge help in applying the Velcro to the inside surface of the pouch, as is being able to insert the magazine inside the inverted pouch.  If you can invert the pouch, just punch a hole for the drain grommet (if present), in the Velcro, carefully slit the peel-off backing along the short axis of the Velcro (centered on the punched hole, and after you have located the hole in the Velcro over the grommet, just peel away the backing, starting at the grommet, and attach the Velcro. Maximum Velcro.

If it's impossible to invert the pouch, and after much trial-and-error, my method is to just cut the Velcro in half and apply the Velcro to the front and back insides of the cleaned pouch, omitting the bottom.  Nip the corners of the Velcro at 45 deg angles to prevent peeling-off at the corners, leaving a new edge of about 3/16". 

The mfrs of the kydex inserts claim that they will "catch" on the insides of the pouch and so not come loose, even if Velcro is not used-- at least on some pouches, sometimes specified, sometimes not.  My suspicion is that this "catching" feature works better on 1000d pouches than on pouches made of thinner, less stiff material.  I admit to being a suspenders-and-belt type, and I install the Velcro.  YMMV. 

I have found that the benefits of the kydex insert are being able to dispense with bungees or flaps, thus giving faster mag access, and much easier single-handed mag insertion into the pouch.  You'll still need some sort of auxiliary mag retention if jumping.  The kydex inserts can, if exposed to enough heat (a car trunk in summer will do it), lose their tension.  In such cases, they can be removed, re-heated, and re-tensioned.  The kydex can be a bit noisy, and will eventually damage the finish on non-plastic mags, and painted plastic mags. 

One other use for 7.62 mag kydex inserts (wider than 5.56 inserts) is to convert a 2x30 rd AR mag pouch into a 1x20 rd 7.62 mag pouch.   Might need to remove OEM elastic bands or bungee originally intended to secure the  2x30rd mags for which the pouch was designed, but it works out pretty well. 

The kydex inserts, like most things, have pluses and minuses.  Try a few out, and see how they work for you.  It may be that you find they are OK for some applications, and not so OK for others.  I can see a person running inserts in some mag pouches and not in others, all on the same rig.

I would make a point of closely examining all the edges of the inserts which might come into contact with the fabric of the pouch enclosing the insert.  To avoid premature abrasion of the fabric, ensure the outer edges and corners of the insert are smooth, with NO sharp edges and corners.  A sharp knife will take the edge off the straight parts, and a file will do for the corners.  600 grit sandpaper will smooth things out nicely.