I made a bike rack which sits on top of the truck bed rails, this allows for the use of the bed plus I can keep my bed topper.
I used steel unistrut material which is typically used for holding electrical conduit and plumbing pipes to cement walls.
You can view my rack
HERE (I will preface that this is a PDF file but the online storage forces you to use their crummy viewer) Additionally I have been having issues with the online storage which if it logs me out the documents are not viewable... So I can not guarantee how long the link will work.
Alternately if you want the use of the bed a quick way would be to fashion a platform which sits on top of the bed rails from 2x4s and plywood, then make a series of frames for the bikes (think like the ones you place the front wheels into).. Then tie down with rubber bungies.