Providing a shallow tray, half covered in pebbles and half with dirt, topped up with water to half cover the pebbles and keep the dirt/mud moist is no problem and covers the water/nesting materials (for mason bees at least. Presumable leaf cutter bees would find enough material in a veg patch for their needs?).
I imagine that would be fine. Some masons and leafcutters are particular about their materials, but many are less so, and I imagine most would be able to adapt to whatever you provide. Leafcutters sometimes prefer material that is a little bit thicker, which is why they love rose petals. I have a ground cover called Creeping Jenny that my leafcutters hole punch all summer!

Some masons will only use native dirt, or prefer a particular consistency of mud, so there could be a little bit of trial and error figuring out what they like, but I can't imagine they'd be picky enough for the whole thing to fail if you didn't have it exactly right.
That leaves food. Before and after the crops provide sufficient pollen and nectar for the bees; would a shallow container of sugar water (what ratio?) provide food for the rest if the year?
Keep in mind that solitary bees are usually not active as long as honey bees or bumble bees are. Since the adults only live for one season, and aren't living in a large, temperature controlled hive, many are not active deep into fall, and many species are only active for a few weeks in the spring or summer, when their favorite flowers are blooming. Put your first tubes out when your first garden flowers are blooming, and leave them up for as long as you have flowers in your garden, because you want to be sure you are catching bees that are active when your flowers are. Definitely plant extra forage crops in your garden (and we can help you with recommendations for good nectar and pollen plants), so there isn't too much competition and you have a little bit of a buffer around your bloom times. Just like with honey bees, sugar is a good emergency feed, if something doesn't go as planned, but it's far better for the bees nutritionally to have real nectar. There isn't a lot of information about feeding other bees sugar water, so I'd be hesitant to do it, but it would be better than having them starve.