Remove the Circuit Playground from the USB cable and plug in the lipo battery. You can wrap it in some tissue paper or foam, and slip it back into the deck with a few cards in front and back -- this will work just fine. However! The weight and feel will be pretty suspicious. Too light, and too squishy!
So, instead, I recommend cutting out some card stock or cardboard to accommodate the electronics, similar to a hollowed out book. Or, you can take some thin plywood to the bandsaw or scrollsaw or a hand coping saw to create a similar insert.
I designed an insert to be cut from 3mm acrylic on the laser cutter. You can download the file and use it on a laser cutter, mill, or even print out on an inkjet or laser printer as a template for cutting cardboard or other materials.
Measured with calipers, the thickness of these five layers comes up just shy of a regular deck of cards, so I shimmed it by adding eight cards to the insert, four on top and four on the bottom.
The additional cards also bring the weight up to nearly identical between the "gimmicked" deck and that of a regular deck of cards.
You'll now reverse the process. When you're ready to use the Freefall Deck (you'll have plenty of battery life, so this can be set up hours before needed) insert the electronics, insert, and any extra cards for padding into the box.
Now, you can use a thin bead of hot glue to close the cardboard flap, and then do the same for the cellophane.
Next, take the Freefall Deck for a spin.