I Need Recommendations

Bones are too soft still and the beetles will eat most of it. Wet specimen is better.

 

Pure formalin is actually about 37% formaldehyde gas dissolved in water with some methanol to prevent polymerization. Assuming you obtain pure formalin dilute this solution 1 part formalin to 9 parts water to get 10% formalin which is used to preserve the specimen. The best manual for you in Spanish is circular 2 at the SSAR site https://ssarherps.org/publications/books-pamphlets/herpetological-circulars/ if you have a dropbox account it will down load this manual which is a translation of the English version.
Pisani, G.R. and J. Villa. 1974. Guia de tecnicas de preservacion de anfibios y reptiles. Herpetological Circulars No. 2.

 

I did a salamander in epoxy resin for a friend of mine. Have you ever tried encasing in epoxy resin?

 

How long a specimen remains in formalin depends on the size. A very large turtle if injected well might need a couple weeks in formalin before transferring to ethanol, whereas a small salamander just a day or two. The specimens should be injected with the diluted formalin mixture and soaked in the same, making sure you position it how you wish. Transferring to 70% ethanol (ethyl alcohol) if often in a stepwise manner, especially with soft bodied salamanders and frogs. If you go directly into very strong alcohol, it will often shrivel or shrink the specimen. Better to go stepwise - overnight rinse in plain water, the next day switch to 40% EtOH and leave for a few days, then into 60% a few days then finally into 70-75%. Each change will extract some of the formalin solution and remove this solution which is acidic and can ultimately destroy the specimen if stored forever in formalin. Sometimes collection do buffer the formalin for long term storage if the specimens are to be sectioned for histology.