87F is cutting it close for heat but is do-able, although temps will get higher as volume decreases with plant growth and transpiration.
72F outside would mean your light and ballast are generating ~15F of heat. That seems about right for your tent size and lighting. Your bulbs are generating a majority of the infrared heat so you should be thinking of it in the simplest terms possible "Hot air rises, needs somewhere to go" So you're not getting the heat out fast enough. Your fan seems sufficient are you running straight duct or with bends (reduces airflow) and where are you venting the exhaust?
Also what kind of RH are you seeing in your tent compared to ambient (this gives us a better glimpse at how efficiently our ventilation is working) ?
Before investing alot more money you'll want to get those clip on fans you mentioned and angle them in such a way that they push the air around and up off your canopy. Again the idea here is to keep air moving continuously from intake to exhaust and exchanging the hot air from your canopy with cool air from your intake, if I read correctly, you just have an exhaust alllllll the way up top of that 6 foot tent with 6 feet of hot air below it, and nothing giving it a boost to the top. I think doing so will give you a good drop in temps.
Indoor summer growing has it's challenges. and there's lots of us here that grow and have grown in temps as high as yours, myself included.
My current setup is a good example, I have a 2x2x4 with ambient of around 78C but even with adequate ventilation and LED it's still ~84C inside.
I also have a 7x7 room with 1000w HPS and temps get 90C+ in the summer. By the way this is with a remote ballast outside the room.
It will get better as summer turns to fall and the days get cooler too, which will line up with your delicate flowering phase.
Happy growing, be sure to share your adventures with us as you get your grow underway!