You can trellis any watermelon within reason - though some will argue otherwise (use common sense). Nature made the plant able to climb and hold the weight of it's fruit. Have faith in this.

There are many materials you can use but I recommend only two after years of trying everything else: cattle panel and Gardeneer Nylon Trellis Netting.

4 x 16 foot Cattle Panel runs $14 at a farm store
They can cut it for you if you need them to with bolt cutters. It will outlast you.

Trellis netting runs $7 - $13 depending on size. It has held a 55 pound melon with no problem. It should last at least five years depending on whether it is protected over the winter. Longer if treated with a UV protectant.

{{gwi:1266356}}{{gwi:1266358}}{{gwi:1266360}}

The cheapest way to put up trellis netting is with electric conduit piping from any home improvement store. You can make a frame with elbows for the corners (they screw into each other) because the conduit is threaded.

Then put the frame over rebar which is pounded securely into the ground or firmly attached to the bed.

Here is one with bent couduit and cattle panel:
{{gwi:1266361}}