I am in the same situation. Because a hd department refused to give me a letter in desired format I used the following theme:
1)An official letter (with a letterhead) from the hr department which includes information about positions and the periods of the employment.
2)2 referenced of a white paper(without an letterhead, he refused to sign with it) from my direct supervisor (the deputy head of a department).
Each of them include the same first paragraph about the name of the positions I held, working hours (20 and 40 respectively) and the periods of employment .
Next, in each of them was written duties which I performed in an each position,
And they were sighed by him and I included his business card (includes contact information).
3)The last one were pay slips, in my case it was employee earnings certificates where it was mentions that my employer was my tax agent and include information about my income.
I really hope that provided information will be enough for them.