I don't have a suggestion but do have a similar problem with my raspberries. I'm not sure a trellis would be suitable though. If yours are anything like mine they have spread out across the depth of the bed so I couldn't tie them all to one trellis.
Here are mine from a couple of years ago.
I think the only solution is a couple of stakes at either end (and intervals in between, if necessary) and string or wire to hold the canes up. Mine tend to flop forwards away from the retaining wall behind and it makes it difficult to spot ripe fruit with all the dense canes in the bed.
Well, it's difficult for me to pick the ripe fruit but the blackbirds have no problem!