This is easy: Ken-Tec SUB2000.
Get it in a Glock mag config that matches what ya got already. Inexpensive and tons of fun!!
Folds in half; fits in a backpack. 16.25 inches folded.
https://www.keltecweapons.com/rifles/sub-2000/
i just picked one up in 9mm Glock for when were out and about in the desert on minibikes. its the size of small laptop.