How to practice alone for a presentation?

Perhaps you are practicing the wrong thing. If in fact you have no trouble speaking smoothly when you are front of a live audience, then you don't need to practice that. You may need to practice how you 'enter' a slide (what concept or thought you will discuss first), or how to transition to the next slide (what is the final concept or thought, and how does it connect with the next thing you want to talk about). You may need to practice what words you say in the introduction... you don't want to make that up on the fly.

In other words, do what professional musicians do. They rarely play an entire piece through during practice, but instead repeat the challenging sections.

With the goal of working out the words you will use in the tricky parts, practice will feel less artificial and more productive. Don't sweat the pieces you already know. At the end, do a dry-run with an audience to test the timing and the flow.

You could try a virtual reality app. There are some for android. They run on Google Cardboard and, some run on Google Daydream as well. If you have a headset already, you might give this a try. I have not tried properly myself yet because the apps were a bit buggy. But, the software provides an okay approximation of an audience. Moreover, there is plenty of psychological support on the efficacy of virtual-reality to help overcome fears (e.g., public speaking). Not suggesting you have such a fear, but I think half the reason we practice public speaking is to make ourselves confident.

For my last practice run, I either ask my group to use them as test audience or I "set the stage". As I guess you already asked everybody and now nobody has time to listen for the third time, let's try and set the stage.

We have a so-called "seminar room" in the vicinity which is rarely used for teaching (actually my own teaching is currently the most regular thing in there). This means it is free to use a lot of the time, especially if you come in early or stay late. I go to this room and set myself up. There is a presenter and if no one is in the room I would sometimes open the windows that I am facing while presenting, so I could imagine "the world" is listening.

Sometimes if I am lucky some students are in there, passing time between lectures. Many know the room is often empty and relaxingly silent. I explain to them what I plan to do and invite them to listen. They are often shy to really give advice, but someone is there and I imagine they might take home something even if they did not listen closely.

So, I try to at least "set up a full presentation stage" and maybe I can attract some spontaneous audience.