When I worked in the MEMS field using LIGA process, we used SU-8 photoresist for high aspect microstructures. The biggest issue concerning the thickness you want to use is maintaining columnar structure. We often did multiple layerings to get thicker build up, but run a higher exposure to error in alignment.

If you have access to a synchrotron you could have LIGA moulds made in the thickness you want, though costs could be prohibitive.