Which risk free rate is appropriate?

[archived_user]

Which risk free rate for analyzing potential equity investment (specific company shares and not specific time horizon) for portfolio with long time horizon ( >25 years).
Important points are:

• The portfolio is located in developing and small country
• There are 25-30 years government bonds (with relatively high yield) but the market is not liquid and I cannot buy big amount of these bonds. For every new issue (2 per year) there is 3-4 times more demand than offer.
• There are 10 years bonds with (significant lower yield -1,5-2%), but with reasonably developed market.

If you know any study or theoretical conclusions, please send me the links