I say it can be done. Just save an old protein jug to do the mixing. Give it a good shake, pour it back, and your all set. You might get some settling out but it is a pre-workout, not a pharmaceutical. If you end up with 90% mix distribution it is still good enough. Now it is a cost verses benefit call. If you are on a budget you can save some coin by buying in bulk, and rolling your own. If cash is not a problem, then a pre-made is the right call.
Personally I am a cheap bastard, and I like to save some money buy adding some too a pre when I want. I also like the single tube approach to a pre-workout. When the gyms were open in my area, I was thinking lifts, sets and reps. Measuring and mixing multiple ingrdiants before a work out I would find distracting. Perhaps working 60 hours a week and still trying to get to the gym might have something to do with this mind set.