This is getting way too hot, regardless of where it’s installed - indoor/outdoor, who cares (seriously guys?). Nice setup, but it would serve you well to purchase a USB-powered 5v fan - plug it into the open USB port on the left to disperse the electrical load (those fans typically consume 1 watt) and mount it to blow over the heatsink you already have installed, or even better if you can mount it somewhat diagonal to also move some air around under the Pi. It will really help to keep some air moving over the heatsink.
It would also help to rearrange the ribs on the heatsink (as it sits) to be more on a north/south arrangement to assist in the chimney effect - hot air is being trapped in the current arrangement in the heatsink rendering it somewhat useless. It’s a free and easy change in the meantime.
So far as indoor/outdoor - too many variables, both are fine. I live where it usually won’t get below 37c (100f) for about 4 months of the year and typical daytime temps are above 44c (112f), don’t underestimate the power of moving air - whether it’s in an enclosure or not - I have enclosure setups in the direct sun here and never a problem. A few bucks will fix you right up, I promise (as big as you can correctly mount is key):
https://www.aliexpress.com/item/32564966542.html
Also, check this thread out - fantastic software controlled fan setup thats cheap: