I agree with MrBreakfst . The decision to use tempered glass for ssff case is not functional is more for atx case with watercooling systems , RGB . You can make some grille for front similar to your design case or take some ideas from HP :
Hp Omen External GPU - look front grill design for airflow also the back side of panel is very nice and clean, HP Z2 Mini , HP Elite Slice .
If you decide to make window panel make it more functional ( fan speed and temperatures CPU/GPU make it from transparent LCD display) look Paul Tan six years before he made very nice project call Reventon PC ( here is some youtube link )
and here is some Antec Cube Razer Edition from GGF Events internal screen from ebay , it need only to make software look from 4:56
And the last one is iBuypower's Snowblind case ( here is official webpage
https://www.ibuypower.com/Signature/Snowblind ).
For 299 USD you must put more technology not just ssff with aluminum/windows panels, its not hard to make all this in reality if you decide make something unique this is some ideas which is nice looking for me.