The Dow company produces a range of solvents called Dowanol, some of which are used extensively in reed diffusers. The most useful are:-
Dowanol TPM (Tripropylene Glycol Methyl Ether) the least volatile
Dowanol DPM (Dipropylene Glycol Methyl Ether)
Dowanol PMA Glycol Ether (Propylene Glycol Methyl Ether Acetate) the most volatile.
Using a mixture of these should be suitable for most fragrances designed to be used in reed diffusers. There is usually about 20.0% fragrance and a mix of Dowanols. Hope this is of use.