Nice work. I do think 15mm is the sweet spot for cost, size and storage. How sturdy is your current table and how much space is there around it? As an option might be to place boards on top, either MDF sheets to the size you need or else insulation boards. The later can be quite lightweight. Of course, this then leads to the issue of where to store the boards when not in use, but it does mean you can still use the existing table for other things.
