OK...here goes...there is more to overwintering Monarchs in the fridge. . . . Firstly regardless of the fridge type...frost free or not...the fridge desiccates. The whole process of refrigeration works by evaporation and all moisture will be collected and automatically removed from your fridge, this will kill your stock. Wait until the autumn...and use the last Monarchs you breed to overwinter them. These will have experienced falling daylight levels and be more suitable to overwinter than those taken earlier in the season (The earlier season Monarchs will expect to pair and lay more eggs!)
The key point is to get a completely sealed airtight plastic container...line it with a little netting for a good grip. Do not add any moisture...this will cause a mould and kill the butterflies. Then get an inch thick Styrofoam box for the airtight box to fit into....this will insulate the plastic box inside and will not subject the Monarchs to temperature fluctuations every time you open the fridge door. The aim is to mirror the overwintering stock in their natural conditions...but do not worry about daylight at this stage...it is not important during complete diapause. Now you generally will lose a few...but very few if you keep the conditions right. It does take a bit of work/time to do it right but it is always worth it to start early.
Above is a photo of a feeding rig you should use...it is a simple construction made from clothes pins in seconds. You should use this to hand feed the Monarchs...to start with you may need to carefully unroll the tongue with a quality paintbrush (I use a Red Sable hair brush size 2..they are not cheap...but they are very springy and retain this feature for ever!) After a few feeds they actually learn and expect what is coming...they even start to unroll their tongues as soon as you put them on the clothes pin. Let them feed...and when they withdraw their tongue.... unroll it again to see if they'll take more...they often do!
Now it is important that they digest their food...so if you put them straight back into the fridge they will be unable to do so...keep them on the peg (This keeps their wings in good condition)...to start with on the peg...they will struggle with their legs but soon settle down. Then leave them for at least 4 hours to digest the food. I use a fructose mix at around 10% at this stage...always add a drop of Soy sauce. Place them in the airtight box put them in the fridge inside the Styrofoam box...to get more Monarchs into a small box....you can do this in stages...so put as many as you can in the box until they start to become a problem...put the box in the fridge and wait until it cools...you can then bring the box out and add more. They don't mind being very crowded so the box does not have to be very large at all....think how crowded they are in their natural overwintering sites! This way you can leave them for around 10 days and start again...after about a month...leave them out for a little longer and double feed them...so feed them as usual...leave them for 4 hours or so...feed them again and leave them for a further 4 hours and put them back in the fridge.
Keep this up all winter and they'll come through with very few losses. Keep an eye on the bodies and any that look a little thin at any time you feed them in the winter...double feed them ...If you start with around 20...you should get around 16 through...I have managed to get them all through with this method. In the spring they may take a week or two to pair and start laying...this is because they are expecting to migrate and there is a delay mechanism built in to allow them to do this and disperse before laying eggs. Good luck and I hope this helps.