Well the best way to fix has been said, remove enclosure pieces. It’s nearly impossible to get enough sealing that it won’t bring dust in like pretty much any sxs. And the heat, well your sitting on the engine and the more enclosed the hotter it’ll get. Like said fan under seat will help.
If you’re keeping everything together you need to find a way to pressurize the cabin with outside air to help some. A scoop/vent that’s open in the forward direction on the roof or an open front window to cram fresh air in. Sealing helps a little but you need pressure
We rode with a guy in Idaho that was hell bent on running full cab (even though it was quick to pull off) on a 100+deg day with only side windows open. Needless to say he didn’t last long at all and went home early. We all stayed enjoying the ride.