You can look into "nail varnish" racks. They typically are simply trays assorted into either a sloped rack or a vertical shelving unit. They are cheap (mass produced) and also because they use a tray rather than a set drilled slot or curve to put the paint in, you can put different kinds of bottle next to each other. Do check dimensions of the trays when they are listed for sale.
I bought one of these the other day
https://www.ebay.co.uk/itm/Nail-Polish-Rack-5-Types-Clear-Acrylic-Organizer-Jewelry-Display-Stand-Holder/264410106083?ssPageName=STRK%3AMEBIDX%3AIT&var=564048170759&_trksid=p2057872.m2749.l2649
Easy to build, holds a lot of paints (I got a 6 rack though in hind sight could easily have got a 7). If you've got limited space though you might opt for a more shelving-unit design rather than a slope since a slope rack has the downside that the space underneath it can be "wasted" unless you can house it over the top of something else.