Razor,
I found these old posts that might be of some help, I haven't read them thoughoughly yet so I dont know how good they are.
1.
the cheapest way is to use house guttering cut into 2ft lenghts and pussed into the beach at 45 degrees then hollow out a space underneath the shelter created,but beware a lot of beaches will not like you doing this,and also all the good marks that will allow it will already be being used by other anglers/bait collecters,the crabs go under the pipe for shelter while the shed,so you need to check them on a regular basis,hope this helps.
2.
all i use is guttering or the down pipe and dig them in the mud. i have got about 600 traps and all of them are a mix of guttering and down pipes. as you are not aloud to put tyres in any more.
3.
Old tyres are probably the best - though not practical in most locations.
The other thing is drain pipe cut length ways or roof coping tiles these should be securely dug into in the sand/mud ( so as not to get washed away in the tides )at an angle of about 45 degrees.
Traps have to checked and cleared regularly to work efficiently - it can also take a few weeks for crabs to appear in new traps.