Playpen climber - Any suggestions?

Hi everyone!
10 days away now from my due date and it seems that all these little monkeys in my care are beginning some monkey like behaviours now.
My own son (23 months) has started getting into his poopy diapers if I don't get to them right away. That I can deal with.
One of my dcg (22 months) climbed out of her playpen today during nap. I always check on the kiddos about 15-20 after I put them down for naps to make sure that they are asleep. She was asleep in her playpen when I checked. I went to wake her from nap today and she was out of her playpen in the ballpit - fast asleep! Stinker!
I asked mom at pick up today and yes - she started climbing out of her playpen this past weekend not her crib yet.
Any suggestions aside from switching her style of playpen or maybe putting some crisco on the arms of it? I really am not prepared to have a child wandering during nap when I desperately am needing naptime to have a rest myself and will have a new baby shortly.
Thanks!

I've seen at Safety Superstore something that looks like a mesh that covers cribs (I am not sure if they have something similar for playpens). It is suppose to cover the whole top of it to prevent kids from climbing out.

I cannot wait until a kid is old enough to switch them to cots. I usually do that around 15 months. I do not want to keep holding them when they start to get heavier and playpens use too much space. I do the same with highchairs. I hate them since they use too much playroom space. As soon as they are able to get in and out of child size chairs by themselves I get rid of highchairs.

I had a child climb out of her playpen as well...she wasn't quite two yet either. She only did it once because she fell and it scared her!! I got a video baby monitor to help prevent that in the future. She was still in a crib at home so her parents put mats around it just in case she did it at home.

Could you buy pool noodles and cut them to fit the sides of the playpen and then slice them open so they fit around the edges? It would give some more height and maybe make it more difficult for her to climb out of?

Buy a sleep sack and put it on backwards, so she can't get the zipper undone, and her legs won't be able to climb.

Thank you everyone!
I knew that this stage was coming but was hoping that it would wait until AFTER my baby arrives when I have more energy to deal with this kind of thing.

I had one able to climb out at 22 months and I just put him in and shut the door and as soon as I heard his feet hit the ground I swung the door open and in a very stern voice and my mean face I said " no climbing out" and I plunked him back in and I continued to do this and after one week he never climbed out again .... You have to be firm and you have to startle them ..... Even if nap time is over you put them back in and then go back a few minutes later and take them out so that they understand they never get out by themselves .

I also use sleep sacks and put them on backwards so they can't climb out.