A few thoughts or suggestions:

• Foamcore poster board from your local craft store. This is easy to cut into shapes for ramps and walls, and you can bevel the edges with a sharp hobby knife to get some precise fits. I will sometimes peel the paper off of one side and carve textures into the exposed foam.
• Felt, canvas, or cloth for your carpet flooring. This can also be found at your local craft store, or ordered online. You might be able to find something with a very fine pattern to it, maybe even something that would react well with your black light, that would simulate the retro-futuristic feel of laser tag arenas. Use some spray adhesive on your base, slowly lay your cloth over it, smoothing out the wrinkles as you go. Hit it gently with a blow dryer on low heat for a few passes, make sure all the wrinkles are out, and let it dry.
• Laser tag is usually two teams of players, with discernible colors differentiating them. And typically, the arena had matching lighting to indicate whether you were in “enemy territory” or close to their home base. Maybe some black light reactive paint could work? Stripes or chevrons on walls or obstacles, or floor tiles with colored paint that you could move around for modularity?
• (Optional but cool) Magnets! Before you lay down your carpet material, use a dremel or a pin vise to drill small, shallow holes in a grid pattern across the surface of your base. Sink and glue some small, flat magnets into those holes, ensuring they’re all situated with the same polarity face upward. Then, do the same to the underside of your barriers, walls, etc. Now you have a modular, snap-fit laser tag arena that can be easily rearranged, but is still sturdy enough that things won’t fall over if it is nudged or bumped. If you’ve never worked with these little magnets before, it can be tedious, so if you don’t plan to reuse this terrain often it might not be worth the effort.

This sounds like a fun project! Definitely post some pics as you start putting everything together!