Hi Ahku,
Below is my DIY crap trap. It may take a couple of days if crabbie is spooked during initial attempt to reach the bait. So a little patience is required. Glad to report that my tank is now crab free.
Things you need: A glass medicine bottle (with opening big enough for the crab to enter), a piece of stone, a rubber band, and a small piece of bait(I use a small part of market prawn head)
Step 1: Secure the bait to the stone with the rubber band. This need to be small enough to fit thru' the bottle opening.
Step 2: Fill the bottle with tank water & slowly lower onto a predetermined spot on the LR or sand bed near to where your crab hides.
Important: Bottle needs to be tilted so that crab cannot climb out after entering the bottle.
Below is my rough sketch. Hope it helps.