Assuming you have an alcove tub, I wouldn't try to add a deck to it--that's just extra complication/cost and doesn't increase the space in the tub, if that's the concern. In the photo you show, that's a style of tub intended to have a deck.
Do you have a photo of your actual tub? That would help. What is the particular issue you're trying to address? If the problem is that water is spraying out, I would first try to adjust the shower head and see if that's the problem. Our 29" tub has a curved shower rod with a standard curtain (though it's curved not due to the tub's width, but because there's a window there that it needed to clear). That works pretty well. Not sure I see the value of adding glass guards to that--more to clean, and at least in our shower, it would block access to the controls at one end. I'd really try to get to the bottom of the splashing problem, if that's the main issue.
You could certainly remove the tub and put a shower in, but that is a big/costly project. If it doesn't otherwise need immediate renovation, I'd live with it first to see how you like it, especially given that it's a guest bath so presumably won't get daily use. (I'm also assuming there's another bathroom in the house with a tub--if not, I wouldn't take this one out.) ETA: I'm also remembering that my in-laws' tub, which is a 28" classic Kohler, does have a glass door. It's a framed sliding door. I didn't love it when our kids were young because it made it a pain to sit on the side of the tub and bathe a child, but it works fine for showers and for baths now that the kids are older and don't need help.
Q