You're lucky you made it to 21 months...Both of my babies started climbing at 7 months and were walking by 8 months. It was crazy to see such little bald babies get up and run around at 9 months. My girls could climb out of their cribs and did. It scared me to death. I just knew they'd fall and crack something and I didn't think a net was a good enough solution. Both of my babies were drawn to any sort of challenge and most baby locks, gates, and safety items were no match for my little engineers.
I didn't want to switch to toddler beds either. However, with my oldest we switched after I woke up one morning to my little one saying "momma! momma!" beside my bed. This was a heart stopping event-the reason why is because my bedroom is downstairs. Hers is upstairs. She not only had climbed out of her crib and over the safety gate in her bedroom doorway but had also climbed over the gate at the top of the stairs AND THE GATE AT THE BOTTOM OF THE STAIRS. Maybe it was overkill using so many but safety is so important but after that-I figured that it was more dangerous to climb the baby gates than just use the stairs normally so we tried a toddler bed.
I hated it. It was a peuny structure that I couldn't really sit on beside her-and kneeling wasn't comfy to me either. After just a few days I got rid of it. In it's place-we placed a normal twin sized mattress on the floor. Once she was used to that-we moved the mattress onto a trundle bed but left it "down" for a long time. Eventually I knew she was used to staying on the mattress and popped up the trundle bed. I was paranoid about her falling off-but the bed was up against a wall for the head and one side and we placed a very plush rug on the open side just in case. She never once fell out. With her bedroom upstairs-she was never in it except to nap or sleep the night. Just because I was paranoid-we rigged the trundle so that it could NOT collapse even if you were able to press the right areas to make it do so. I always figured that if I could think of it happening then it could happen. Besides, I have toy dogs too-if not my child then maybe my little dogs could be smashed.
My second child wasn't as determined to escape as my oldest but once she started climbing otu there was no keeping her in. It was no different than taking her back repeatedly and having her lay back down...
I don't think a net would have worked well-I did consider it for my second but then my crib wasn't a uniform height and I didn't want to purchase a new crib just to use a net when she was already climbing from the crib.
Instead-to keep her in bed it meant that either I, or my husband, had to stay with her for 15 or 20 minutes each night until she was asleep. In the grand scheme of things this wasn't that long really. I tried to go about it logically and not spend the time rubbing her back or smoothing her hair or holding her close or whatever. I was just THERE to make sure she stayed in bed and stayed quiet. (I do a LOT of loving the rest of the time and just before bedtime. I wanted to be sure my girls could learn to go to sleep on their own-and not just be put to bed already asleep.)
This lasted a lot longer for my second child than my first as my second has a different body clock than my first. I adjusted by keeping a good book handy. I'd lay down-and use my book light to read and my little one would snuggle up to my back. Sometimes I stayed longer than the time I had to just because it was sweet listening to her even breathing and feeling her little hand on my back or hearing her automatic thumb sucking...sometimes I got lost in my book and stayed longer because I was reading. It worked just fine and both children were used to sleeping on normal twin size beds up the normal height by the time they were two.
This made stays anywhere very easy. We didn't have to make pallets on the floor or worry about hotel room beds or scramble to find a crib or bother with a pack n play. What's the point when they can climb out anyway? I didn't have to worry about them waking up in the center of someone's bed and falling off-because they wouldn't fall off and they knew very well how to climb off by then too.
My babies are now 7 and 4. There are even better toddler beds availible now than those metal or plastic types but most use the crib mattress still. I guess it wouldn't matter unless your child is a wild sleeper like mine were. I still recall both babies wacking their heads-arms or legs on the slats of the crib because they rolled over and slammed into it-WHACK! on the montior. Sometimes it woke them and they cried-mostly it just woke my husband and I up ona nd off through the night. BLECK.