I think I have the orthography for Montreiano figured out:

a - /a/
b - /B/ or /b/
c - /k/ before a, o, u
 - /ts/
cu + vowel - /kw/: cuatro
d - /D/ or /d/
e - /e/
 - /e:/
f - /f/
g - /k/ before a, o, u
gu + vowel - /g/: guia
g - /gw/
h - nil: haueo - /auweo/
i - i
i + vowel - /j/: gemma > iema
*j - This I am not too sure about. I thought of using it to represent /h/
in loans, but I am not sure.
*k - /k/
l - /l/
ll - /l_j/
m - /m/
n - /n/
 - /N/: e /eN/
i - /N_j/: anno - aio
o - /o/
 - /o:/
p - /p/
qu + vowels - /k/
r - /r/
rr - trilled r
s - /s/
t - /t/
u - /u/
u + vowel - /w/: uanacaste
v - /B/
*w - /w/
x - /S/
*y - y
*z - /s/

Accents: generally, for the long vowels, , and , the stress usually
falls on them (because of their length). If it falls on another vowel,
that vowel will be marked with an acute accent (dont ask me for examples,
I cant think of any). Also, for cu, if it represents /ku/ instead of /kw/,
then the u will be accented: ca. The same applies to gu.

Letters marked with asterisks arent generally used in Montreiano, but are
found in loans (I cant think of a use for j, but maybe one of y'all can
help me think of one) .

B and d are special cases, because some speakers (like me :)), will
sometimes say b as /b/, instead of the usual pronunciation of /B/, and d
as /d/, instead of the usual pronunciation of /D/ (hey, the start of
dialectal variation ;))