I think this one is the bee's knees but perhaps less useful for users of outboards...also involves paying for a kit but the plans can be bought separately...
and here's a page from a brochure where the real marine plywood (not to be confused with the sub-standard stuff in most timber merchants) is still sold.
This lot will also cut a sheet of ply to any shape you like.
BS1088 is a standard that refers only to the methods of construction, not the species of timber used. The required standard is surprisingly low and there a a great deal of variance between suppliers. Few offer anything like the standard that existed before BS...
Cheap has its virtues; it would be very easy to spend a lot on the more desirable qualities to the point that before very long you might wish you had bought something and saved all the grief too (unless the grief is the fun part).