How can I enclose this shower?

Add magnetic weights to the bottom of your shower curtain. Sandwich the curtain between the two. You probably could make do with 10 magnets. I have dual curtains, a hotel style fabric liner and the decorative one to make the room pretty.

You can buy shower curtains with the magnets already put in.

I hate to tell u this, but you would/might get lots of water leaking on to the floor. I have one made of individually placed marble like tile. I took the easy way out🤣👏👏 I put up a shower curtain and it hangs inward. It works great. Just remember to make sure your curtain has room to hang inward. That will stop water from leaking inwardly. Much cheaper than a door too. Plus most shower doors open outward. Can cause a wet floor every time u shower.

I put up a shower curtain. I let the curtain hang inside of the door. I have a shower step that goes inward. Stops water from getting on the floor. Also allows u to just turn the shower head towards the curtain and rinse it towards the curtains and quickly rinse it down to dry during the day. That is the cheap way to cut corners. Magnets basically r not going to work in as much as water will find a way to splash. It will not work on that shower material. You can go to the expense of having Home Depot or Lowes put in a track and a door. That will work and prevent water. Do check to see if a sliding or outward door can be installed. If u have an outward facing door, check. Many times the kind of shower u have pictured needs an outward facing door to open. Make sure that the track is secure or it will leak.

"shower rite" glass doors

If you sew a length of interconnected bead type weights into the hem of your shower curtain liner, it will not billow out. I strongly recommend the fabric type shower curtain liners, for ease of maintenance, vs. the vinyl type. You can simply open one seam end to remove the beaded weights, if laundering becomes necessary.

That unit is fiberglass and magnets won't work. It's either a glass door or weights or suction cups. Custom glass doors can be expensive but probably the best solution as they have a gasket that is sealed along the bottom. You can have them make the door open into the shower or bypass would work also.

Having only one panel that moves to open and close like a door will be much less expensive than a full enclosure with a separate framed inside door. The trend now is to have only one wide panel covering part of the shower opening and no door, but as your shower is very narrow, this may not provide enough of a 'wall' from the spraying water.

The picture on the left below shows two narrow panels attached together; the one on the right functioning as a door. The picture on the right shows one style of 'hinges' that attach to both panels.

https://www.build.com/dreamline-shdr-3630720/s1283703?uid=3051545&source=gg-gba-pla_3051545!c1655532377!a64347491435!dc!ns&gclid=EAIaIQobChMI29Sk9Pji3wIVlsVkCh37-AkqEAQYASABEgKTwPD_BwE&gclsrc=aw.ds

Here are some more examples of trackless shower panels.