I like these:
http://m.lowes.com/pd/AcuRite-Digital-W ... on/3830957
I stick a chopstick up into the hole on the back, and then set them around the tank in the substrate. I have two in my 90g and one in the 10g iso. I also have smaller temp and humidity gauges on sticks or bits of plexi (that was me that posted that idea about gauges on sticks, it's been a lifesaver--literally--for us). Temp and humidity is so essential for crabs I always have multiples so I can cross-check (even if they've been calibrated, things can go kaflooey with them). Having them on sticks instead of stuck to a tank wall allows you to quickly set up an iso with some gauges you know are working properly, if necessary.
I really think that putting them on the tank sides is not the best thing, anyway. The glass can conduct heat, so you aren't really getting an accurate read of what the ambient temperature is inside your tank, but something influenced by whatever the air temp is in your house as well, and water vapor often condenses on the glass, so the humidity is likely to read higher right there than it really is in the ambient air in the tank. Being able to stick your gauges in the substrate here and there throughout the tank gives a much better read of what the conditions are where the crabs actually spend time. Further, you can move them when you like, so you can check different areas, and they don't have to limit your decor options, either (once you stick them to the tank wall, you're "stuck," pretty literally--you can't put a UTH there, you can't put a climbing wall there, you can't put climbing apparatus too close or they'll pull them off the wall, etc., etc.).
Using chopsticks and putting them in places where they aren't right next to climbing apparatus, I don't find the crabs climb on the gauges at all (I think a chopstick is too narrow for them to think it is a branch to climb on). And those Acurite gauges just happen to be at a great angle to read from the top of a tank, with a chopstick stuck up into their "hanging hole" on the back. They aren't super cheap, but they aren't crazy expensive, either. So far (I've been using them since before Christmas), they've been working great.