Based on the OP's location, that's the standard range rules for any indoor range in the area and the closest outdoor range is likely 90+ minutes away..
If you're allowed to do mag changes, practice mag changes. But as others have said, accuracy work, accuracy work, accuracy work. When I was stuck at ranges like yours, I worked on breathing control and sight alignment. Ball & dummy drills. Un-timed Casino Drill. Finding my balance and resting position with the gun at extension. This may sound dumb, but shooting with my eyes closed (helped to see if I was keeping control of the gun and driving it back to where it previously was or if it was wiggling everywhere).
I also bought a little laser cartridge that I could put in my Sig and a little Laserlyte reactive target that I used at home. In total, it was around $120. Since I wasn't able to practice from the holster at the range, I made sure I practiced draw and DA trigger pulls at a reactive target when at home after the kids were in bed.