Hi apartment strata-doomed friend! Have you checked these out? They sit inside on your window sil - they may not create a full 100% seal height-wise but they keep most bugs out and cats in! They stock some sizes at Canadian Tire if you hate Amazon.
You can add in a window gate (they make metal ones) or a double screen (like one of the cheap portable ones that slide open like this one - https://www.amazon.com/dp/B00004Z13K/ref=cm_sw_r_apan_i_GK74VA6H35HGHN3CQNY8 - although you should beef it up with hot glueing the screen in place). That gives you an extra layer of protection.
Get one of those adjustable sliding screens, (WB Marvin Frost King AWS1037 Adjustable Window Screen, 10in High x Fits 21-37in Wide https://www.amazon.com/dp/B00004Z13K/ref=cm_sw_r_cp_api_i_GVA910G9W58BVF4BMQ71)
WB Marvin Frost King AWS1037 Adjustable Window Screen, 10in High x Fits 21-37in Wide https://www.amazon.com/dp/B00004Z13K/ref=cm_sw_r_cp_api_i_WYHY4ZXQP26H9HCRSJPS
These just set in your windows and are a super cheap solution!