So the kayaks are simply sliding in from one end, just sliding along the shelf surface?

Cheap material depends on what is cheap locally, and cheap is a relative term. Scrap sheet metal or other panel material may exist.

In some places the standardized marine shipping containers can be purchased for modest money. These are made of heavy steel, with substantial lockable doors at one end. They can be customized as needed then delivered and relocated using normal container trucks.

The standard shipping container length is 20 feet, I think, but there are several sizes of extended containers.

If a shipping container will work, the 'shelves' might become simply a series of rollers or round rods that the kayaks slide over.