Maybe just put cardboard on the bottom
It's cheaper and they like chewing cardboard. A hamster's sole purpose seems to be to chew out of any enclosure! However big. If he had the whole room he'd probably try to get the door open to get out! But then that's all stimulation for them.
He sounds to be having a great time in the playpen with you! Do you think he is fully hand tame now? I used to progress from the playpen to the sofa. And have sofa time. Obviously you need to be there all the time. The more clutter on the sofa the more they enjoy rummaging and hiding under things. Our last syrian used to climb the back of the sofa and walk along the top like an intrepid explorer - I had to stand there just in case he tried to jump and steer him back down with my hand. Mostly I'd just pick him up when he got to the top and put him back on the sofa again. And have a hide on the sofa they can retreat to and have a wash or something. They can climb all over you there as well (up your sleeve, down your back, lol).
But it depends if the hamster is tame enough. If not tame they'll just speed off and throw themselves off it! So I used to put a barrier (a big blanket) along the edge of the sofa with one syrian - it slowed him up enough that I could get him before he jumped.