Glass tanks are fine, but still have the same problem as your enclosure. The open top. Trying to heat and humidify an open topped glass tank is like trying to heat your home in winter with no roof and no ceiling. You need a closed chamber. You have to close off the air inside from the air outside, just like our homes. You also need the heating and lighting inside the enclosure instead of sitting on top creating a chimney effect.
The other problem with glass tanks is that they are too small for anything but a tiny baby. Even a 100 gallon tank will be too small for your baby in a few months if it is growing normally. They can work well for starting babies if you can close in the top.
I chose the title of this care sheet very carefully. Are there other ways to raise babies? Yes. Yes there are, but those ways are not as good. What follows is the BEST way, according to 30 years of research and experimentation with hundreds of babies of many species. Babies hatch during the...
you can easily fix your Guinea pig cage to be a closed enclosure by using bread ties (zip ties?) and attaching sheet plastic to the outside of the wire. Attach with ties only to the sides and back, and use an extra piece of plastic just draped over the front so you can lift it to get into the cage. It wouldn't be as secure as a real closed enclosure, but it would work ok as an intermediate measure. Then pour a pitcherful of water over your substrate to wet it, and each week, or as needed, add more water.