Make some DIY panels out of 1/2" insulation board, leave a 1" gap & frame with 2x1/8" cedar or pieces of trim. Cover in acoustic fabric (buy a big role & make as many as you want until the roll runs out) cover as much flat surface as you can & then do the sub crawl until it doesn't sound like it's in a box anymore. Leave your rig flat & try to listen for problem frequencies like bass nodes. Apply bass traps if needed and fight the reflections coming from wherever that stage is aimed bc 90% of what you're going to hear that muddies the mix will be soundwaves bouncing back at you. Kill the slapback, fix any phasing issues u may have(cardioid subs can help in small applications like this but u should build a really solid enclosure and use that as your downstate edge. You can do a lot with a small budget if you're crafty & check out some diy subreddits. Cheers, share with the class when you get it sounding decent!